| What happens if you put it in but not quite all the way, i.e. about 1 mm away from fully-seated? If the problem goes away, you might have only a spring contact that's lost its tension, and you might be able to recover it by going in there (through the jack opening) with a bent coat hanger or something, and pulling on the spring contact to bend it so you get more tension out of it. CAREFULLY!
If that's not the problem, maybe there's a cold/broken solder joint at the jack, assuming it's PCB-mounted. And I could be wrong on that last point; I've never been inside a 700.
Phone jacks do have a "max # of insertions" spec. Also there are cheap jacks and good ones. Some of the offshore knock-offs don't hold up all that great. I have no idea what the 700 uses.
Sorry I cant do better than guess at the problem. | 
