Issue with Teams Client on IOS making a call but no 200 ok return from MS when answering the call

Hey Guys,

We've recently setup teams calling and are at the testing stage.

running into an issue with making calls form my ios client to a physical desktop phone in the company.

When i make the call from the teams client i see my line light up on my physical phone to show that its in use. the line which i'm dialing starts to ring and i pick it up on the physical phone. The physical phone seems to pickup the line fine but my teams client on IOS just continues to ring to the extent that it starts ringing again on the physical phone again..

we've run wiresharks and looked at the firewall to make sure all ports are open as needed but what we noticed is that we don't receive a 200 ok back from MS.

now we have two SBC's and i believe the certificate was created with both of them in it.

FQDN's are correct and all licenses are fine..

If someone calls me internally or from an external line i can answer the phone fine on my IOS teams device.

Anyone come across this issue as I'm really stuck and have opened a case with Microsoft a couple of days ago but they haven't even picked it up yet..