Call going silent after being transferred to external phone number in Auto Attendant

Hi there,

I have an Auto Attendant set up in Teams with an option that redirects to an external phone number.

When I press the option that redirects my call to an external phone number, I hear "please wait while your call is being transferred", then a few rings, then silence. I've tried different formats for the number, in case that is what is wrong, e.g.

• 07912345678
• +447912345678
• 7912345678
• +4407912345678

All go silent. The resource account for my Auto Attendant has the following licenses:

• Microsoft 365 Domestic Calling Plan (120 min)
• Microsoft 365 Phone System - Virtual User

I noticed that if I place the call using my Teams client (because I also have the phone system license), the call is put on "hold" when it goes silent, here is what it looks like:

Does anyone know what I am missing? I've tried diverting to a mobile and a landline but both have the same result.

Regards,

Christian