# Airtel (via your SBC)

Source: https://docs.usetone.ai/sip-trunking/airtel

> Front an Airtel enterprise trunk with your own SBC and hand the calls to Tone.

*Airtel's enterprise voice trunks usually terminate on customer premises equipment — an AudioCodes,
Ribbon, Oracle/Acme or Cisco SBC in your data centre. Tone connects to **that SBC**, not to Airtel
directly, and this page is about the SBC hop.*

**Not yet validated end to end by Tone against a live Airtel trunk.** Tone-side fields are exact;
the SBC guidance is generic because it depends on your make and model. If you run this, tell
support@usetone.ai what you hit.

## The shape

```
Airtel PRI/SIP  →  your SBC  →  Tone gateway  →  your agent
                   (you own this hop)
```

Your SBC keeps doing what it already does toward Airtel. You are adding one more SIP peer on its
other side. That means:

- **Nothing about your Airtel contract changes.** No new provisioning request, no new DIDs.
- **The SBC is where number formats get normalised.** If Airtel hands you `0`-prefixed or bare
  ten-digit numbers, your SBC almost certainly already normalises them for your PBX; do the same
  toward Tone.
- **Your SBC's public address is what Tone allowlists**, so it must be static.

## Before you start

- An SBC with a public IPv4 address and capacity for another peer.
- The DIDs you want a Tone agent to answer, and a plan for which calls route to Tone versus your
  existing PBX.
- A Tone organization with KYC verified and **SIP trunking** enabled by Tone support.

## Configure the peer on your SBC

Whatever the vendor, the peer toward Tone is:

| Setting | Value |
|---|---|
| Destination | `sip.usetone.ai` |
| Port and transport | `5061` TLS — or `5060` TCP/UDP |
| Codec | G.711 A-law (PCMA); PCMU as fallback |
| DTMF | RFC 2833 |
| Media | RTP, UDP `16384–32768`, to and from Tone's IPs |
| Registration | none |
| Request URI | the DID in E.164 (`+919876543210`) unless you set another format in Tone |

Allow Tone's signalling IPs — on your trunk's page in the dashboard — inbound on your SBC, and
allow your SBC to reach Tone outbound. Open **both** SIP and the RTP range; a firewall that passes
SIP but not media produces a call that rings, answers, and is silent, which is the single most
misleading failure in this whole setup.

## Create the trunk in Tone

```http
POST /v1/sip-trunks
{
  "name": "Airtel via DC SBC",
  "inboundAuth": "ip_acl",
  "ipAcl": ["203.0.113.32/32"],
  "outboundHost": "sbc.yourcompany.example",
  "outboundPort": 5061,
  "outboundTransport": "tls",
  "outboundNumberFormat": "e164",
  "callerIdPolicy": "did",
  "codecs": ["PCMA", "PCMU"],
  "maxConcurrent": 30
}
```

`ipAcl` is your SBC's public address — a single `/32` if you have one SBC, or the pair if you run
an HA cluster that can egress from either. Private ranges are refused, so the address must be the
one Tone actually sees, not the SBC's internal interface. If your SBC is behind NAT, the allowlist
entry is the **NAT's** public address.

Import each DID:

```http
POST /v1/sip-trunks/{id}/numbers
{ "e164": "+911244567890", "agentId": "…" }
```

## Routing only some calls to Tone

Most people do not send an entire Airtel trunk to an agent. Because routing lives on your SBC, you
can send one DID, a range, or a time-of-day condition — and keep everything else going to your
existing PBX. Tone only ever sees what your SBC sends it, and answers only the DIDs you imported:
anything else gets a `404`, which is a useful safety net while you are still moving dial plan
around.

## Testing

1. `POST /v1/sip-trunks/{id}/test` — `OPTIONS` from Tone to your SBC. Proves the outbound half and
   your firewall, before any real call.
2. Call one imported DID from a mobile.
3. `POST /v1/calls` with that DID.

If step 2 rings and then goes silent, it is media, not signalling — check the RTP range in both
directions before anything else.

## When it does not work

| What you see | Why |
|---|---|
| Inbound `403` from Tone | Your SBC is egressing from an address not in `ipAcl` — common with HA pairs and with NAT. |
| Inbound `404` from Tone | DID not imported, or imported on a different trunk. |
| Inbound rejected, number format | Your SBC is sending `0…` or a bare ten-digit number in the request URI. Normalise to E.164 on the SBC, which is the right place for it. |
| Ringing then silence | RTP blocked one way. Tone's media is UDP `16384–32768` from the same IPs as its signalling. |
| Outbound fails at Airtel | Caller ID. Airtel validates CLI against the numbers on your circuit; keep `callerIdPolicy: "did"` and use a DID Airtel knows. |
| Codec negotiation failure | Force A-law on the peer. Tone's gateway offers PCMA first, but an SBC pinned to something exotic will not meet it. |