Prepend the Area Code with Asterisk
If you want to prefix the area code for 7-digit numbers with Asterisk, it usually looks like something like this:
ext => _NXXXXXX,1,goto(505${EXTEN},1)
But if you need to server all area codes dynamically, you can do something like this:
ext => _NXXXXXX,1,goto(${CALLERID(number):0:3}${EXTEN},1)
This assumes caller id is set, and that your callers are using standard PSTN-like DIDs.