As I understand it, paging systems have a satellite and terrestrial component. So, you are still at the mercy of land-based transmission systems and local paging company owners. So, I suspect the reason they were able to page us in Key West is because we were a) docked in the U.S., b) close proximity to a terrestrial paging transmission tower. I wouldn't assume that paging would work at other ports of call. However, our AT&T wireless cell phone did work in Cozomel. Paging system and wireless/cellular system coverage is one of those great mysteries, you can only be certain it works by testing it out.
Regarding cost: we don't know yet what we will be billed for our call to the Oceaneer's club. We were given a direct phone number to call, but I haven't seen the bill yet.
VS