Calls from Microsoft Teams App compared to Teams web app

Hello 

Bit of a tricky one but I believe it maybe a network issue. I have a situation where the client calls a business contact, goes through to an IVR, press's the required selection and after getting put through to the agent all audio drops from the call. 

Strange aspect is, if you make the call with the Teams app, it works, make it with the Teams web app and it fails (I have tested this by calling from two numbers that go over completely separate carriers). I have captured network logs and can see a continuous stream of UDP so I know the call is not failing, it seems the audio process is being dropped when being routed through the webb app. 

Has anyone had this before or would know the difference route/process a teams app phone call makes to a teams web app? I believe it is probably a network issue on the B Party side, but as with most calls, proving that is half the work.

I am going to do my own research but not come across clear yet, if anyone has anything to add that would assist in this I would be grateful (as well as if I come up with the answer I will put this down as it is an inconvenience for the client).