How does international texting/calling work in El Salvador?

I have a friend who is currently visiting family in El Salvador. I’m aware that they don’t have any or much cell service unless you pay for it in that area but she has an American phone so just uses wifi to text and call when she’s at her family’s house. When she leaves the house with her phone my messages or calls don’t deliver/go through at all.
It’s just weird because I spoke with her last about 2 weeks ago now and my Calls and messages are not going through HOWEVER- every few days or so, my message will deliver. Just not often. When I see it delivers I try to send another one and the next one does not go through again…. Is it possible that she’s out and gets enough spot of service just for my message to deliver and then it goes away again? I just worry that’s all and I’m not very tech savvy. But I’m assuming that’s what happens. She’s not home with her family so she doesn’t have wifi for my messages/ calls to go through but every so often wherever she is- gets maybe a small spot of service and it will go through? Then back to no service.