exten => 888,1,Set(MYVAR=4165551212)
exten => 888,n,GotoIf(${REGEX("[0-9]{10}",${MYVAR}) = ${MYVAR}}?yup)
exten => 888,n,Playback(auth-thankyou)
exten => 888,n,Hangup
exten => 888,n(yup),Playback(vm-no)
exten => 888,n,Hangup