I have never used it, but I think this is what it's used for....
Look into DISA. I believe if you have DISA set up you can dial a call as if you were connected to an actual system. So I would assume you set up DISA on the 3800, you dial the DISA code of the 3800 from the 3550 and then you should get a dial tone and be able to place the call out the 3800.
These are generalities - there is probably more to it. I do know that DISA is considered dangerous if not properly secured, because anyone that has that code could conceivably call it and have unrestricted dialing access.
Som of the other experts will either give you more information or tell you I'm FOS, but I believe that would be one way to do it. I know I have it set up on my systems so that I can dial a code at the main site, say 77, and in my dial plan is has "77" routed to the site in question. At that site, "77" is set up so that it dials the overhead paging trunk. That method is a little more secure because it has a specific action defined, and is not wide open. Hell - if you go that route and you have open spots in your dial plan you could configure "8" in the dial plan of the 3550 to route to the 3800, and on the 3800 you could have "8" configured to work the same way as "9".
