It is already specified as IMS (IP multimedia system), and it is requirement for the "next generation" phone standard, LTE. However, I don't see how that is going to make life better security-wise that GSM voice. SIP is notoriously insecure protocol (hello please route phonecalls unencrypted to this IP, trust me because here is my password in plaintext). Also reliability and battery life is worse, so there is valid questioning in the industry on "why bother?".