How to generate crypto key and encrypt/decrypt audio on SIP call with TLS 1.2?

I have a custom SIP phone that is working without TLS 1.2 on RingCentral but I want to update it to be more secure. Can you help me with the algorithm to generate the crypto inline key that I need to send on the INVITE request and the encrypt and decrypt method for the audio bytes? I have a SIP phone that we have programmed in Java.