Assuming the 456 is the ATA number and the outside number is always 10
digits.
exten => _456.,1,Dial(SIP/${EXTEN:0:3}/${EXTEN:-10},tT)
but then it might as well be
exten => _456.,1,Dial(SIP/456/${EXTEN:-10},tT)
> ; to dial outside thru GSM gateway
> exten => _456.,1,Dial(SIP/${EXTEN},30,tTD(${EXTEN:3}))
> exten => _456.,n,Hangup
>
M