I have heard the CDMA phones don't have SIM cards, and the phones that are GSMA do have SIM cards , I've also read on my computer if the code letters that end in a C letter if they run on AT&T & Verizon towers, and if the codes have a letter G they work on T-Mobile, maybe Sprint ??Towers, Straight Talk pays for the cell towers they use. Anyway some share the network towers for better area coverage. I have also read a lot of conflicting info. so it's better to go to the Straight Talk web site and pick your topic at the top to scroll through. I'm sorry,... you can type in the difference between GSMA & CDMA cell phones, I've even typed in area coverage maps from straight talk web sites........ by: apple top I hope this answer helped some people with other Questions also.
Yes, it does not matter what network the phone you're calling to is on. Your call will go through no matter as long as your son is in an area where HE has reception. And if he is not, it will go to voice mail.
You shouldn't worry at all what network the number you are calling will be on.