I've been using my Virgin Mobile sim card with an iPhone 5s with absolutely no problems. Now I switched to a new iPhone 6s and the phone says it's an "invalid sim." I went to Apple and had the phone switched for a new iPhone 6s after they checked the phone, found nothing wrong with it but assumed that it must be a hardware malfunction. The new phone recognized the sim and showed Virgin as the provider in the left upper corner of the screen. However, when I got back from the store the phone no longer recognized the sim card, displaying "invalid sim" in the left upper corner of the home screen instead of the provider "Virgin."
What could be the problem here? Is my sim card incompatible with the new iPhone? I think it's very unlikely that there's anything wrong with the phone after having tried the sim in two different iPhone's 6s. Btw, I just put the sim back into my old iPhone 5s and there it's working perfectly.