Regional routing - Cartesia Docs

Twilio phone numbers are scoped to a region through the API credentials used to manage them. The region determines how Twilio routes API calls, and matching it lets Cartesia serve region-specific traffic from a nearby cluster.

How it works

When you import a Twilio phone number into Cartesia:

Twilio routes the call regionally. Twilio directs incoming calls to the region configured on the API key that manages the number: us1, ie1 (Ireland), or au1 (Australia).
Cartesia must target the same region. To act on a live call, such as a transfer or hangup, Cartesia sends API requests back to Twilio. These requests must reach the region where Twilio is handling the call.
The region on the provider keeps both sides aligned. When you set a region on your Cartesia provider, every request from Cartesia targets that Twilio regional edge.

The region is tied to the provider’s API credentials, not to the phone number’s country code or your agent’s region. A UK number (+44...) under a us1 provider routes through a US Twilio server. To route it through the closest server, import it under an ie1 provider.

If the region is missing or wrong, transfers and outbound calls fail with authentication errors. The region on your Twilio credentials must match the region on the Cartesia provider.

Configure the region

Check your Twilio region

In the Twilio Console, go to Phone Numbers → Manage → Active numbers, select your number, and check the Edge Location under Voice & Fax.Supported regions: us1 (US, default), ie1 (Ireland), au1 (Australia).

Set the region in Twilio (if needed)

If the edge location does not match the region you want, update it under Voice & Fax and save.

Non-default regions require a region-specific API key from Twilio. A standard US key does not work for ie1 or au1.

Match the region in Cartesia

When you create a provider or import a phone number, set region to match your Twilio configuration.

Set up multiple regions

If you operate in several Twilio regions, create one provider per region:

In Twilio, create a region-specific API key (see Step 2 of the integration guide).
Link the same Twilio Account SID once per region:

# US region
curl -X POST "https://api.cartesia.ai/agents/phone-numbers/providers" \
  -H "X-API-Key: $CARTESIA_API_KEY" \
  -H "Cartesia-Version: 2026-08-14" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "twilio",
    "account_sid": "YOUR_TWILIO_ACCOUNT_SID",
    "api_key_sid": "US_API_KEY_SID",
    "api_key_secret": "US_API_KEY_SECRET",
    "region": "us1"
  }'

# Ireland region
curl -X POST "https://api.cartesia.ai/agents/phone-numbers/providers" \
  -H "X-API-Key: $CARTESIA_API_KEY" \
  -H "Cartesia-Version: 2026-08-14" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "twilio",
    "account_sid": "YOUR_TWILIO_ACCOUNT_SID",
    "api_key_sid": "IE_API_KEY_SID",
    "api_key_secret": "IE_API_KEY_SECRET",
    "region": "ie1"
  }'

Each Account SID and region pair is unique. Import each phone number under the provider that matches its routing region.

Troubleshooting

Symptom	Cause	Fix
Outbound call fails with an authentication error	The API key’s region does not match the provider’s region	Set the provider’s region to the region the Twilio API key was created for
Calls route through the wrong region	The number was imported under the wrong provider	Update the number’s provider with `PATCH /agents/phone-numbers/{id}`
Credential update rejected with unreachable numbers	The new API key cannot access the numbers in the provider’s region	Use a key with access to every number. Twilio lists the unreachable numbers in the error response

​How it works

​Configure the region

​Set up multiple regions

​Troubleshooting

How it works

Configure the region

Set up multiple regions

Troubleshooting