I have Verizon and do as Kim said, turn off cellular data and turn off airplane mode. It does not initiate the $10 per day Travel Pass automatically. Last time I was on SXM, it charged $1.79 a minute for talking, regardless of who initiated the call. Received tests are $.05, sent texts are $.50. I also do as she does and wait until I am back at the condo for calling (using wifi calling). Depending on how urgent it is, I may respond to texts immediately or wait until I return to iMessage. There may be a difference in some plans as I recall working with them years ago to get the plan that worked best for me. I typically end up with a bill between $10-15 for 2-3 weeks. Well worth it for me to be able to communicate with kids, grand kids, and dog sitters for my geriatric dogs.

I have pretty good coverage around the island I think, but I'm not using the phone much while I'm out.