exten => 702,1,Goto(myphone,1)

exten => myphone,1,SetCallerID(${CALLERID} |a)
exten => myphone,2,Dial(SIP/9702,25)
exten => myphone,3,Goto(100)
exten => myphone,100,Voicemail(9702@from-pstn,u)
exten => myphone,101,Hangup