SIP Gateway

Regional SIP gateway between ElevenLabs and customer SBCs — static IP, in-country presence, media anchoring.

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Problem

graph LR
    LK[ElevenLabs SIP] -.->|"❌ Dynamic IPs<br/>❌ Outside country<br/>❌ No FQDN support"| SBC[Customer SBC]
    style LK fill:#fee,stroke:#c33
    style SBC fill:#fee,stroke:#c33

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Issue	Impact
SIP server IPs are dynamic	Legacy SBCs (Five9, etc.) can't whitelist
SIP INVITE originates outside country	Blocked by local telecom regulators (e.g., Turkey)
SBC requires IP, not FQDN	Direct connection impossible

Solution

graph LR
    LK[ElevenLabs SIP] -->|TCP 5060| GW["<b>SIP Gateway</b><br/>Static IP • In-Region"]
    GW -->|SIP + RTP| SBC[Customer SBC / Twilio]
    style GW fill:#dfd,stroke:#0a0,stroke-width:2px

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Deploy a lightweight SIP gateway in any region. It provides a fixed IP, in-country presence, and media anchoring — all RTP audio flows through it.

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Two Approaches

🔹 Kamailio + RTPEngine Stateful SIP proxy with dedicated media relay. ~2ms overhead per call ~1000+ concurrent calls Automatic header passthrough Zero-copy RTP forwarding Best for: production, high scale, transparent relay

🔸 Asterisk B2BUA Full Back-to-Back User Agent (PBX engine). Full codec transcoding support IVR, DTMF, call recording at the gateway Asterisk dialplan flexibility Best for: PoC, when you need call logic at the edge

Comparison

	Kamailio + RTPEngine	Asterisk B2BUA
Latency	~2ms	~10-20ms
Max concurrent calls	1000+	50-100
Header passthrough	Automatic	Explicit mapping
Codec transcoding	✗	✓
IVR / call logic	✗	✓
SIP dialog	Preserved (proxy)	Re-originated (B2BUA)
Config complexity	~150 lines	~200 lines

Which one?

flowchart TD
    A{"Need transcoding, IVR,<br/>or recording at gateway?"} -->|Yes| B["<b>Asterisk B2BUA</b>"]
    A -->|No| C{"Need >100<br/>concurrent calls?"}
    C -->|Yes| D["<b>Kamailio + RTPEngine</b>"]
    C -->|No| E["Either works — Kamailio<br/>is lighter for pure relay"]
    style B fill:#ddf,stroke:#33a
    style D fill:#dfd,stroke:#0a0

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Call Flow

sequenceDiagram
    participant EL as ElevenLabs SIP
    participant GW as SIP Gateway
    participant SBC as Customer SBC

    EL->>GW: INVITE sip:+1234@gateway:5060
    Note over GW: Rewrite R-URI → customer SBC<br/>Anchor media (RTPEngine/Asterisk)
    GW->>SBC: INVITE sip:+1234@sbc.customer.com
    SBC-->>GW: 100 Trying
    GW-->>EL: 100 Trying
    SBC-->>GW: 180 Ringing
    GW-->>EL: 180 Ringing
    SBC-->>GW: 200 OK (SDP: SBC media IP)
    Note over GW: Rewrite SDP → gateway IP
    GW-->>EL: 200 OK (SDP: gateway IP)
    EL->>GW: ACK

    rect rgb(240, 255, 240)
        Note over EL,SBC: 🔊 RTP audio flows through gateway
        EL->>GW: RTP (UDP)
        GW->>SBC: RTP (UDP)
        SBC->>GW: RTP
        GW->>EL: RTP
    end

    SBC->>GW: BYE
    GW->>EL: BYE

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Quick Start

GCP Deployment

Kamailio + RTPEngine

cd kamailio-proxy/terraform
cp terraform.tfvars.example terraform.tfvars
# Edit region, zone, project

terraform init && terraform apply

cd ../scripts
./deploy.sh --customer-sbc your-trunk.pstn.twilio.com

Asterisk B2BUA

cd asterisk-b2bua
PROJECT_ID=your-project \
TWILIO_TERMINATION_HOST="your-trunk.pstn.twilio.com" \
bash deploy-gcp-middleware.sh

On-Prem / Non-GCP (Turkey, etc.)

Kamailio + RTPEngine

sudo bash kamailio-proxy/scripts/install-onprem.sh \
    --external-ip <PUBLIC_IP> \
    --internal-ip <PRIVATE_IP> \
    --customer-sbc your-trunk.pstn.twilio.com

Asterisk B2BUA

sudo bash asterisk-b2bua/install-onprem.sh \
    --twilio-termination-host your-trunk.pstn.twilio.com \
    --external-ip <PUBLIC_IP>

Both scripts install Docker, write all configs, open firewall ports, and start the stack.

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ElevenLabs Configuration

After deploying, configure the outbound trunk:

Field	Value
Address	<gateway-ip>
Transport	TCP
Auth	Username/password (if enabled)

Twilio Elastic SIP

Direction	Configuration
Termination (outbound)	Add <gateway-ip>/32 to IP Access Control List
Origination (inbound)	Add URI sip:<gateway-ip>:5060;transport=tcp

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Requirements

Resource	Minimum
OS	Ubuntu 22.04+, Debian 12+, CentOS 8+
CPU	2 cores
RAM	2 GB
Ports	TCP 5060, UDP 5060, UDP 10000-20000
IP	Static public IP (or NAT with port forwarding)

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Project Structure

sip-gateway/
├── kamailio-proxy/              # Kamailio + RTPEngine
│   ├── docker/                  #   Docker Compose stack
│   ├── terraform/               #   GCP infrastructure
│   ├── scripts/                 #   Deploy + on-prem installer
│   └── README.md
├── asterisk-b2bua/              # Asterisk B2BUA
│   ├── deploy-gcp-middleware.sh #   GCP provisioning
│   ├── install-onprem.sh        #   On-prem installer
│   ├── startup-script.sh        #   VM bootstrap
│   └── README.md
└── README.md                    # This file

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License

MIT

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_{Built for ElevenLabs Conversational AI telephony}