I also have US Cellular, and US and Verizon are roaming partners, meaning, if you can't get a US tower, you roam (in your area) to a Verizon tower (and visa versa for a Verizon customer). The problem is, if your phone can barely receive a signal off a US Cellular tower, it will not roam to a much stronger Verizon tower (and visa versa). For example, Verizon brought Rural Cellular, with a tower on Smith hill in Holderness. It serves Squam well, but if you try to call on a USCC phone, it will barely hear the Moultonboro towers and fail trying to place a call on them whereas the Verizon tower is 3-4 bars.
I think the problem is its all economics, technology could permit you to use the strongest tower (such as dialing 911 does), but the companies don't make money paying roaming fees
