I seem to have a probleme lately with sending text messages. Receiving text messages and phone calls works perfectly. The problem is inconsistent, some days everything is working just fine and some other days i have to turn airplane mode on/off, when i get the "message not sent" error, several times in the same hour. What could cause this? Rebooting the device also fix the problem temporarily. This also happens on/off wifi. I am using the regular text message app. Every contact is affected by the problem.