; try this macro

[macro-SipPhone]
exten => s,1,GotoIf($[foo${CALLERIDNUM} = foo]?5:2)
exten => s,2,GotoIf($[${CALLERIDNUM} = ${ARG2}]?3:5)

; do this if the extension matches the callerid
exten => s,3,VoicemailMain(${ARG2})
exten => s,4,Hangup()

;do this otherwize
exten => s,5,Dial(${ARG1},20,m)
exten => s,6,Voicemail(u${ARG2})
exten => s,7,Hangup()
exten => s,106,Voicemail(b${ARG2})
exten => s,107,Hangup()


;and call to it from extensions.conf like this...
exten => 102,1,Macro(SipPhone,${EXTEN},${VoiceMailNumber})


;where VoiceMailNumber is the number of the vm box for that extension