With Sprint, if you have service in those areas such as neah bay or 40 miles off Westport, you are roaming on another carriers network, garunteed.
Mooch, thats called "accidental roaming" and happens quite frequently in situations where you are bordering a partner carriers service area. Most all carriers will re-rate your bill if you call into customer sevrice and wait on hold for 1 1/2 hours
I work for AT&T wireless, and will admit Verizon has better coverage all over the Olympic Pen. Verizon and it's affiliates have a larger customer base in these areas, so it makes sense for them to invest in coverage. There are alot of other factors to weigh in, I wont get into all of them. The largest is this. Installing new towers to cover areas like that all comes down to return on investment. AT&T has not yet installed towers that will cover those areas because the amount of customers the towers will potentially cover are not cost beneficial. Thats just the way carriers see it.
One thing I have done in the past when fishing in obscure areas is switch my mobile service on to my 3 watt bag phone. That way I have 5x the transmitting and receiving power over a handheld. For anyone who wants to rely on cell service in obscure areas for periods of time, this is the way to go.
You can pick up a portable bag phone at most pawn shops for cheap, noone wants them. Once you have your mobile # programmed into it, you can easily switch your service back and forth between your handheld and bag phone by calling into your carrier and switching ESN's. Alot of commercial fishing boats in the Gulf of Alaska still use these phones and can place calls up to 100 miles offshore from a cell site.