How does CryptTalk interact with GSM calls?

In iOS, GSM calls take priority over everything else.

If you receive a CryptTalk call while you are in a GSM call, the CryptTalk call will automatically be rejected.

If you receive a GSM call while you are in a CryptTalk call, you must first either accept or decline the GSM call before returning to the CryptTalk call. Everyone else participating in the CryptTalk call will be put on hold and notified that you are in a GSM call.