Apparently they have them in all the stores for the various providers. It would be a bit embarassing for them if you couldn't get signal in their own stores.

I don't think you'd be able to complain to your provider about signal levels when they're pretty good, but I had luck at contacting T-Mobile. They claimed to have great service in my area, but it was really so bad that my phone couldn't even find a network. After several calls to customer service and some visits to their stores, I got word that they'd be looking at a site to add service.