/etc/asterisk/extensions.conf

[macro-dialGSM]
exten => s,1,Dial(SIP/${ARG1},20)
exten => s,2,Goto(s-${DIALSTATUS},1)
exten => s-CANCEL,1,Hangup
exten => s-NOANSWER,1,Hangup
exten => s-BUSY,1,Busy(30)
exten => s-CONGESTION,1,Congestion(30)
exten => s-CHANUNAVAIL,1,playback(ss-noservice)
exten => s-CANCEL,1,Hangup
[sip-external]
exten => 2000003,1,Macro(dialGSM,IMSI3102XXXXXXXXXX3@127.0.0.1:5062)
exten => 2000004,1,Macro(dialGSM,IMSI3102XXXXXXXXXX4@127.0.0.1:5062)