With United phone res (and the other majors) you reserve a class of seating (Y,B,M,H,Q,V,W) when you book, and are quoted a fare, but if the fare is withdrawn from the market before you ticket you are out of luck.

I once booked SFO-Boston-SFO round trip at a 7 day advance fare of under $400. When I went to ticket the next day that fare had been changed to 14 day advance; the cheapest 7 day advance was $1200. I was lucky....as Premier Exec the res agent waived the advance purchase reqt and gave me the $400 fare if I E-ticketed on the spot. Lesson learned....