Hi!
This is one of the ways to do a loopback trunk/route as dicko suggested:
Now if I understood you properly you have the whole range of numbers starting with 918-555-[123]. If so I think that putting 918555 in the prefix and [123]XXX in the match pattern (where I put the DID in my example) would most likely work as the prefix would be removed when the number is dialled leaving only the extension.
Good luck and have a nice day!
Nick