I keep getting a "sim failure" message on my iphone 5. To fix I must turn the phone off completely and then back on, but only works for somewhere between a several minutes and a several hours. In the meantime I miss calls, and since I use the phone as my main business line this is a BIG problem. I have taken the phone back to the Verizon where I purchased it and they replaced the sim card (3 times now), but it is not a permanent fix and on this last return they wanted to charge me $10. Why? What should I do?