Limiting Routes by extension

Question:

I want to limit all my routes so that only extensions 1xx can dial out. Easy enough.

However, I have an inbound route that hits a ring group that rings an external PSTN number (as well as an internal extension). When I limit the Outbound Routes as above, this no longer works. There's no route available to dial out. My question is, how can I get it to work? What "extension" is the Asterisk system using when it calls am external PSTN as above?

Also, can I put in extension ranges in the outbound route selection? eg "101,105,106[-9]"

Thanks