You can always purchase a cellular modem that supports voice, and then plant it on a circuit board with you own choice of microprocessor. I like the ATSAM4Sxxx series of microprocessor or the STM32L476RG. The cellular modem responds to AT commands, just like the telephone modems of old. Surely you can handle the audio circuits after all of that.
You’ll probably be limited to GSM bands though. I dont think 4G is open yet.