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1

Hi,

We would like to connect Retell AI with Proximus in Belgium, specifically with our Cisco CUCM phone system.

Our goal is to:

  • Make outbound calls using a Belgian phone number

  • Allow call recipients to call back and reach our existing phone number

  • Ensure inbound calls are routed through our Cisco CUCM phone system

Our ISP (Proximus) has indicated that we cannot use the SIP option for this setup. Additionally, the currently supported calling options in Retell AI do not appear to include Belgian phone numbers.

Could you advise:

  • Whether it is possible to connect Retell AI to Proximus SIP trunks or directly to Cisco CUCM?

  • What configuration would be required to enable outbound calls with a Belgian caller ID?

  • How we can ensure that return calls are routed back through our existing phone infrastructure?

Kind regards,
Laurens Van Acker

2

I can help clarify your options for connecting Retell AI with your Proximus/Cisco CUCM setup.

Connecting via SIP Trunking

Retell AI supports integration with custom telephony providers through SIP trunking(1). The platform connects to the Retell SIP server at sip:sip.retellai.com(1). This works with elastic SIP trunking from various providers(1).

For your setup, you would configure your Cisco CUCM or Proximus SIP trunk to point to Retell’s SIP server(1). Retell supports multiple transport methods:

  • TCP (Recommended): sip:sip.retellai.com;transport=tcp(1)
  • UDP: sip:sip.retellai.com;transport=udp(1)
  • TLS: sip:sip.retellai.com;transport=tls(1)

You can whitelist Retell’s IP blocks: 18.98.16.120/30 (all regions), 143.223.88.0/21 and 161.115.160.0/19 (certain US traffic)(1).

Belgian Phone Number Configuration

To use your Belgian phone number with Retell AI, you would need to import it using the Import Phone Number API(2). The number must be in E.164 format (including the country code with a + prefix, no spaces or special characters)(2).

During import, you’ll provide(3):

  • The termination URI to identify your elastic SIP trunk (for outbound calls)(3)
  • A unique agent ID to bind the number, ensuring inbound calls route to the specified agent(3)
  • Optional username and password for SIP trunk authentication(3)

Routing Considerations

For your requirement about routing return calls through your existing phone infrastructure, you would need to configure your SIP trunk’s origination settings to direct inbound calls appropriately(1)(3). The setup allows you to maintain control over call routing through your Cisco CUCM system while leveraging Retell AI’s voice agents(1).

Next Steps

Since Proximus indicated SIP isn’t an option, I recommend:

  • Clarifying with Proximus whether they support elastic SIP trunking specifically (as opposed to other SIP configurations)
  • Confirming whether your Cisco CUCM can establish a SIP trunk connection to external servers

For additional support with your specific configuration:

Would you like a general answer based on my knowledge instead?