Note: Paging mode is accessed by pressing the menu key while the phone is off-hook.
In Asterisk, calls are generally dialed using a dialplan command such as:
exten => _2XX,1,Dial(SIP/${EXTEN},25)
To enable paging through pressing the menu key to get into the “Page Using” mode, two lines will need to be added before this command:
exten => _2XX,1,GotoIf($[“${SIP_HEADER(Call-Info)}”=”answer-after=0”]?2:3)
exten => _2XX,2,SIPAddHeader(Call-Info: answer-after=0)
exten => _2XX,3,Dial(SIP/${EXTEN},25)
The first line added checks to see if the phone designated the call as a page call. If it is, Asterisk jumps to priority 2, where Asterisk itself designates the call as a page call; otherwise it jumps to priority three. Please be aware that original Dial command needs to have the priority increased by two, as there are now two commands ahead of it.
Source: Grandstream