exten => 300,1,NoOp(Trying extension 300)
same => n,GoToIf($["${SIPPEER(iphone,status):0:3}"="OK "]?ok:forward)
same => n(ok),NoOp(Dialling extension 300)
same => n,Dial(SIP/iphone,20)
same => n,GotoIf($["${DIALSTATUS)}"="ANSWER"]?:forward)
same => n,Hangup
same => n(forward),VoiceMail(500@voicemail,u)