Backup Calling Guide #272
Replies: 17 comments 14 replies
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I'm trying this on my Samsung S22 Android 14. Nada, expert mode and all. Wi-Fi calling works fine but as soon as it switches over to data, it's done. Not sure wh Google can't get basic stuff to work compared to Apple. Calling sim: Roaming |
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@fillwithjoy1 Forgive my ignorance, but is this patch necessary if you already have the "Switch mobile data automatically" toggle available under your calling SIM? Or is this patch necessary to get that toggle to show up in the first place? |
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Thanks @scrag for your feedback, but it will drain the battery and get the phone heated if it continuesly keep finding for signal....it defeats the purpose, any other way? |
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It will get a signal eventually through a.partner network. I've got it working that way without issue when overseas.
This is how backup dialling works.
…________________________________
From: Paqueta7 ***@***.***>
Sent: Monday 4 November 2024 00:56
To: kyujin-cho/pixel-volte-patch
Cc: Jan Haak; Mention
Subject: Re: [kyujin-cho/pixel-volte-patch] Backup Calling Guide (Discussion #272)
Thanks @scrag<https://github.com/scrag> for your feedback, but it will drain the battery and get the phone heated if it continuesly keep finding for signal....it defeats the purpose, any other way?
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You mean no connection like flight mode? Once before going to the new android version. When I turn on flight mode and WiFi on, it showed my carrier in Malaysia. Then after that it didn't and I can't do any flight mode calls with WiFi turn on. |
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On your Calling Sim you need to select a network that is forbidden so that the Calling Sim has no connection.
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From: Paqueta7 ***@***.***>
Sent: Monday 4 November 2024 13:47
To: kyujin-cho/pixel-volte-patch
Cc: Jan Haak; Mention
Subject: Re: [kyujin-cho/pixel-volte-patch] Backup Calling Guide (Discussion #272)
You mean no connection like flight mode? Once before going to the new android version. When I turn on flight mode and WiFi on, it showed my carrier in Malaysia. Then after that it didn't and I can't do any flight mode calls with WiFi turn on.
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Oh ok, I get it....anyway to use back calling from travelling sim data? |
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That's what backup calling is.
You are making calls from your Calling SIM through the data connection of your Data SIM.
When you make a call it should say something like "calling via 'Calling SIM' shared-data call".
…________________________________
From: Paqueta7 ***@***.***>
Sent: Monday 4 November 2024 14:41
To: kyujin-cho/pixel-volte-patch
Cc: Jan Haak; Mention
Subject: Re: [kyujin-cho/pixel-volte-patch] Backup Calling Guide (Discussion #272)
Oh ok, I get it....anyway to use back calling from travelling sim data?
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@Paqueta7 and @scrag Can we please discuss under this thread. It's making this discussion look very messy. @Paqueta7 Have you read my guide completely? It tells you what you need to configure to get backup calling working. See if you can get Wi-Fi calling working first. |
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Yes @fillwithjoy1 , I have read your guide. but my phone is not working in vowifi, if I turn on airplane mode and and wifi. If works well if i use turn off airplane mode and keep my wifi on, I'm able to call my home country. And also your guide need to be updated a little where i think most people have moved to android 15 and the options are diffrent. Please advice, |
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does this backup calling still works on Android 15? if yes can the guide be updated Q#2: does the Data SIM really need wifi and\or voLTE enabled? it seems that it would only use the data part to make a call. |
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This guide is a literal savior and I share it every time someone asks about using a foreign data sim to act as WiFi calling on a pixel. USA based here. Two SIMs. SIM 1 has no service at one job site. SIM 2 does, but is very slow, but it's enough for me to get backup calling and backup texting. Is there a bool where I remain connected to WiFi, but because wifi ports are blocked, it falls back to Backup Calling as SIM2 has data? I work in the IT department, yet they are incompetent to fix the issue despite me pushing the issue the last two months since it started. I'm on a 9 Pro. |
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European Samsung Galaxy S24 Ultra user here, tried everything but unfortunately nothing works to get backup calling working, it simply only works on WiFi When I desconnect from WiFi it reverts back to "Emergency calls only" |
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So, I know that the actual AOSP codebase specifically mentions that the data sim needs to be in the home network for Backup Calling to work, hence the reason why you have a section in your Troubleshooting section dedicated to talking about the scenario where your data sim is roaming. However, I have tested the following scenarios:
And in both situations, my Pixel 9 Pro XL (April 2025 patch) still actually establishes Backup Calling for my T-Mobile US sim, despite my data sim being on roaming. I don't need to force override It makes me wonder if Google forgot to update the code comments to reflect this new behavior... |
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Hi @fillwithjoy1 , I am using Evolution X based on Android 16 on my Redmi Note 10 Pro in India. I want to enable backup calling feature for both of my SIMs to fix the calling issue when one SIM lost coverage. Is it possible to achieve this using this Pixel IMS workaround? I am new to these things. |
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Hello any way to activate auto switch data sim option? I have OnePlus open Apex edition android 15 and this option is not available in EU version ,but in IN is there, I already activated backup calling but still not working and I believe it because auto switch data sim not active. |
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This is a guide for activating backup calling on your device. Android 14
TL:DR your calling SIM should have "Switch mobile data automatically" turned on. You do not need to turn on Data or Roaming. If the Calling SIM then reads "(carrier name) Backup calling" even if it reports no signal then Backup Calling is on. At this point just check that your data SIM allows wifi calling (if VoLTE works, you should be fine) and then pray that it connects.
For me it is not reliable and I'm occasionally able to get it to work but the success varies heavily based on your country. Just because it's unreliable for me doesn't mean that it won't work for you
Long guide:
Clearer guide on what to do (eSIM, pSIM doesn't matter)
When I refer to the
Data SIMwe are talking about the local sim that has internet availableWhen I refer to the
Calling SIMwe are talking about the roaming sim that has Wi-Fi Calling available (VoWIFI). If your carrier didn't have Wi-Fi calling working (test with airplane mode) before you travelled, this won't work.Calling SIMenable these togglesEnable VoLTEEnable Backup CallingEnable VoWiFiEnable VoWiFi while roamingcosmetic wise, you can enable these
Show VoWiFi preference in SettingsShow VoWiFi Roaming preference in SettingsShow VoWiFi IconOnce you have done these go to your

Calling SIMsettings and enableSwitch mobile data automatically(screenshot provided)You don't need to enable Roaming or Mobile data
Restart your phone
You should observe your


Calling SIMshowing "Backup calling". It doesn't have to be connected yet (screenshots below, myData SIMis Jio and you'll notice calling anyone withCalling SIMreports no signal).Eventually your Calling SIM should connect successfully.
Troubleshooting
My
Calling SIMis stuck on "Backup calling" but doesn't connect in the end.There could be a variety of reasons why it doesn't work. Personally I've found that opening Pixel IMS and going to
Expert Modeon myCalling SIMand turning onFORCE_HOME_NETWORK_BOOLhas started to get it working.Edit: It can also be due to your network blocking Wi-Fi calling (UDP Port 4500, IPsec Pass-Through) 1
My
Calling SIMswitches to "No Service" / "Emergency calls only" after a while (but it did connect successfully before)Try this:
Calling SIMExpert ModeFORCE_HOME_NETWORK_BOOLand set it to trueMy
Calling SIMis saying it's connected to Roaming and yet reports that it's connected as Backup calling.You have Roaming On and it's overriding Backup calling
Calling SIMSIM card page and scroll down to Network sectionSelect Network, or toggle offAutomatically Select Network.Calling SIMis connected to a local network operator.(forbidden). This will force your phone to use Backup Calling as it sees that Roaming is now not available.My
Data SIMis marked as Roaming, will this cause issues?Yes because backup calling only kicks in if your
Data SIMis in the "home network" (aka no roaming) 2For Backup Calling to kick in, your
Calling SIMhas to be in Roaming or Out of Service and yourData SIMhas to be in the "home network". If yourData SIMis marked as roaming, then you'll need to bypass this by:FORCE_HOME_NETWORK_BOOLand set it to TrueI am available for anyone who needs to get this working, just open a new thread and I'll do my best to help you
Outro
It's finicky at times. YMMV. I've noticed that Backup Calling works more reliably with different SIMs and even with different countries, so go figure
Edit: Your carrier plays a huge role. For me my carrier requires me to stay connected to a tower to even enable wi-fi calling, making it nearly impossible to use backup calling effectively. I've got another carrier that doesn't have the same restriction, but keep that in mind
Notes
Footnotes
Wi-Fi Calling ports required ↩
Conditions for Backup calling to kick in ↩
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