How do I use the "chat" feature under android? When I try to usn it I get an error message "Not Implemented (501)"
The error is sent by the server and seems to indicate that SIP messaging is not supported on the server side.
Please talk to your provider or PBX administrator to see if they can add support for chat messages over the SIP SIMPLE protocol
Alternatively you might prefer Xmpp Texting. It can be easily secured with OMEMO.