Using issabelPBX release 49 you can configure pjsip message-context setting. And then you must create a couple of custom context in order to exchange sip simple messages between phones. You could set the message-context to "textmessages" and add this into extensions_custom.conf
[textmessages]
exten => _.,1,Gosub(send-text,s,1,(${EXTEN}))
exten => e,1,Hangup()
[send-text]
exten => s,1,NoOp(Sending Text To: ${ARG1})
exten => s,n,Set(PEER=${CUT(CUT(CUT(MESSAGE(from),@,1),<,2),:,2)})
exten => s,n,Set(FROM=${DB(AMPUSER/${PEER}/cidname)})
exten => s,n,Set(CALLERID_NUM=${DB(AMPUSER/${PEER}/cidnum)})
exten => s,n,Set(FROM_SIP=${STRREPLACE(MESSAGE(from),<sip:${PEER}@,<sip:${CALLERID_NUM}@)})
exten => s,n,MessageSend(pjsip:${ARG1},${FROM_SIP})
exten => s,n,Hangup()
