Here is an example of two paging methods that can be used from the Grandsteam GXP-2000 and the GXP-2020 phones.

The first method uses the Paging function in the phone.
This is the most natural and allows you to use the phones built-in feature.
You select your line and hit the OK (GXP-2000) button or the MENU (GXP 2020) button
This puts the phone into paging mode then you dial the extension.

The second method is to prefix the extension with an * character.
This is the traditional method offered by Grandstream on their site, I have provided them with the first method and they are working on a white paper using the first method.

This example was written to run on Asterisk 1.4.x

[Ext-IN]
exten => _2XXX,n,Set(l_Exten=${EXTEN})
exten => _2XXX,n,Set(l_IsPaging=${SIP_HEADER(Call-Info)})
exten => _2XXX,n,GotoIf($["${l_IsPaging}"="answer-after=0"]?callInterCom|1)
exten => _2XXX,n,Goto(Internal,${EXTEN},1)

exten => _*2XXX,1,Set(l_Exten=${EXTEN})
exten => _*2XXX,n,Goto(callInterCom,1)

exten => callInterCom,1,Macro(CoreExtPage,SIP/${l_Exten})


[macro-CoreExtPage]
exten => s,1,ChanIsAvail(${ARG1}|js) ; j is for dump and s is for ANY call
exten => s,2,SIPAddHeader(Call-Info: answer-after=0)
exten => s,3,NoOp() ; Add others here
exten => s,4,Dial(${ARG1}|j)
exten => s,5, Hangup
exten => s,105,Hangup

- BEZ (zktech) http://www.zktech.com