How to modify a value in asterisk

Asterisk Asterisk Support
1

hi -
In my asterisk dialplain i get the user’s phone number by:
exten => 1,7,Set(phone_num=${CALLERID(num)})
I would like to remove the leading “1” that this variable stores - how can I do this ???
thanks
mjh

2

exten => 1,7,Set(phone_num=${CALLERID(num):1})