Can't make external/outgoing phone calls on Teams

I get this error message when I try to make external calls on Teams. What's the issue?

This message means the phone number you are dialling is incorrect - Usually it has an extra space, an extra 0 or the country code plus a zero E.G +44 (0) 7465 xxx xxx. Often due to copy and pasting a phone number.

Please try to call your own mobile from Teams to confirm you can dial external, and if you still have problems please forward the number you are trying to call to us and we will double check it.