Bandwidth won't be apparent during the handshake. And voice data isn't really that large anyway, though it would be plausible to detect that some kind of stream is going on as opposed to discrete messages. Still, that would have to come mid-call rather than when the call is attempted.

Sure, in this case it's based on what port they are using. But, it's not a winnable problem due to bandwidth and the option to simply block everything. Limit encrypted streams to 3kbps and you can do eCommerce or slowly send a selfie not real time audio.