I can think of 3 options off the top of my head first, just what you said get a gateway Quentum will do a great job you will need a license on your 5000 controller for it to work.
Second you could order a line have call forwarding put on it and dial * code to forward here it is *72 point it to a DID or trunk on your system and away you go.
Third and what I think might be the best all around solution would be a market expansion number, most providers offer this it is a virtual number pointed at one of your trunks.
O.K. maybe 4 but I would say the least desirable if you are trunking customers across it Google Voice it is a free service with local numbers all over the us, you may need an invite to get it (I might have one left)I have it give it out all the time, it will ring where ever you need it to point it to any number local or long distance it will propagate the CID (I use it because it is in my free call list from Verizon on my cell I show the google number as the CID and it is a free call) It has good audio most of the time but not all of the time this is where I think it might not be the best idea or way to go but, never the less an option
Hope all this helps
Randy