Allow me to point out that besides AT&T, US Cellular is the only nationwide player who has a true nationwide plan and a true unlimited nationwide plan. However AT&T requires a 2 year contract if you wish to get on one of their plans. US Cellular never makes customers sign contracts if they go to a new plan.
Verizon, Sprint, and T-Mobile have unlimited to mobile customers or their own mobile customers or within their network. Have a look at their web sites. Not difficult to find all the disclaimers, etc. Also, Sprint is raising their Early Term Fee to $350 to pretty much match AT&T and Verizon (US Cellular's is at $150 with proration). Sprint also announced they double their "Upgrade Fee" from $18 to $36 - that is if you attach a new or refurbish a phone to your line.
If anyone can call themselves truly nationwide it appears to be AT&T, followed by US Cellular. Do your research on the companies own websites.