Microsoft Teams phone edition use b=CT:50000

Bronze Contributor


I have troubleshooting why the outbound calls are failing when using the IP phone (AudioCodes C435HD), but then using the Teams Desktop version calls are working as expected.


When comparing the SIP messages arriving from Teams servers to our SBC (yes, direct routing is in use) I can see the following difference:

Desktop Teams:


Teams IP phone:



Based on RFC: SDP: Session Description Protocol see, the section: "5.8. Bandwidth ("b=")" it is interesting to noticed that IP phones requires over 10x more bandwidth compared to desktop client.


Any have an idea if I could impact to this somehow? The reason why I'm asking this is that our operator end the call and gives:

SIP/2.0 488 Not Acceptable Here

Reason: Q.850 ;cause=127 ;text="Interworking, unspecified"


Yes, I could remove the B=CT line on the SBC, but I would like to avoid customizations as long as possible.




3 Replies

Hi @Petri-X,

I've seen also some BT values with 1.000.000.

To get a working SBC configuration, we are implementing a message manipulation in each of our Direct Routing implementations and adjust the BT value to 1.000.

It works for most of the providers without any impact to the telephony itself. 

Thanks @Thorsten Pickhan, do you have knowledge is this coming such from the devices itself, or is this something what Teams services are defining?


But perhaps I need made few manipulation rules :)


Hi @Petri-X ,

sorry, I don't have any knowledge if the value is set by the device or the platform itself.


Best regards,