NAT Control

It is important when the phone is behind a NAT. Use force_rport, comedia in almost all the cases.
If you experience one way audio, then check the NAT setting.
Qualify allows the PBX to contact periodically the phone to check if it is still online. This has the benefit of taking “open” the connection tracking on the firewall you can have between the phones and the Internet. Connections are made every 60 seconds. If you have slow phones, you can increase the time to wait for an answer.
Keep Alive is another way to take open a connection, using keep alive packets.
RTP Keep Alive permits to take open an RTP connection when no audio is passed when a party is not talking for a long time.