Some of my friends can't send texts to my Nexus 4. The messages either freeze and don't send, or they have to keep trying repeatedly until it gets through hours later. They are all on iPhone, however another friend with an iPhone managed to get through without a problem. When I turn off the WiFi an exclamation point appears over the Signal Bars which are showing full strength. It goes away when I turn WiFi on again. I have restarted my phone, and taken out the SIM card, neither have helped. Any experience with this problem or ideas on how to fix it?