Outgoing calls on specific phone number

I’m fairly new to all of this, so I appreciate any pointers!

I have about 7 phone numbers setup on my Asterisk box. The inbound routes are already setup so that specific extensions are rung if someone calls into certain numbers. Basically a typical direct line setup.

What I want to do is add a custom feature code that will allow an extension to place an outbound call that uses one of the 7 phone numbers I have. Let me draw a quick picture …

main line: 555-555-5551 (all calls outbound are from this number)
additional lines: 555-555-5552 through 555-555-5556

555-555-5552 -> always rings extension 710

when 710 makes an outgoing call, call always goes out from 555-555-5551

I want to make it so when 710 dials a feature code like *999 then dials an external number, the call goes out from 555-555-5552.

If I can’t do it with a feature code I also want to know how to set it up so that outgoing calls from 710 always go out on 555-555-5552, not the main number. This would be useful to know regardless.

I know my terminology is probably horrible, but I’m trying to get a point across.