Danadog - we rented a car from Royal on our recent trip and used the free cell phone they gave us. The emailed us the phone number a day or two in advance so I could give it out before we left. I, too, only needed it to check in with my elderly parents and to have in case they or one of the kids needed us in a emergency. We went to Ling & sons and purchased a phone for about $15, but we had trouble getting the phone programmed and finally asked guest services at our hotel to help and she got it done, but ended up using a lot of our minutes calling the phone card company to get it straightened out. Turns out something was printed wrong on the card or something like that...I never did understand that whole thing.
I was just happy when she finally got it working.
Long story short, we decided we would not go that route again but would instead activate international calling on our own Verizon phones and take them with us. You may not have that option with your cell phone company, and you probably won't have the same problem with your rental phone anyway. Also, that particular phone had very different features than ours, so it was a little frustrating trying to get used to it. Truthfully, we found it very difficult to use. I'm not trying to talk you out of the rental cell, I just wanted to share our experience so you'd have it to consider.
Oh yeah...one other tip, when you are calling the US you dial 001 for the international code, but when friends and family are calling you in Aruba, they dial 011.