What type of phone are you using jtdl? One thing to be aware of is that different carriers use different frequencies, and not all phones offer the same frequency bands. Of the 4 or 5 bands that my Galaxy SIII phone supports (there are many, many models of SIII, each supporting slightly different frequency bands for different carriers), only one is common to and supported by the Roam Mobility carrier (T-Mobile). Now that was still enough to get reliable service at Disney. It is possible if your phone is older, or is a Telus model that doesn't use compatible T-Mobile frequencies, that it is having problems finding an available tower that it can talk to. When you switched back to your Telus SIM card in Vegas, did you notice which US carrier you connected to? It is all a little complicated, but if you research it you can find the frequencies supported by your phone and those used by various US carriers fairly easily with an internet search.
I have now switched to Roam Like Home on Rogers because they roam on multiple networks in the US; that allows more connection options and I notice my phone most often connected on AT&T, although sometimes on T-Mobile (which is Roam's network carrier). Service is better this way, no additional number or worries, and roughly the same cost as Roam Mobility.
