exten => 123,1,GotoIf($[${ISNULL(${CALLERID(ani)})}=1]?NULL)
OR
exten => 123,1,GotoIf($["${CALLERID(num)}" = ""]?NULL)
exten => 123,n,GotoIf($[${LEN(${CALLERID(num)})}<5]?NULL)<br>
exten => 123,n,GotoIf($["${CALLERID(num)"!=""]?GO)
exten => 123,n(GO),Set(isnumber=${REGEX("[0-9]" ${CALLERID(num)})})
exten => 123,n,GotoIf($["${isnumber}" = "1"]?PASS)
exten => 123,n(PASS),NoOp(GO)
exten => 123,n(NULL),Hangup()