After updating both mine and my wife's iPhone 4S from 5.01 to 5.1 a few days ago, we started having a strange issue with both of our phones. When placing a call, instead of hearing the phone ringing, there is sound which I can only describe as "Garbled" "complete digital distortion". When the other party answers, it's the same. We can sometimes place several calls without this happening, but sometimes it will also happen several times in a row. We have good signal (3 or more bars) and it's not related to the signal level as far as I can tell. I thought that maybe it was a a local AT&T service issue and called their customer support to report this. They looked at line quality and such and then asked if I had updated to 5.1 and I told them I had. She then said they had been getting a large amount of calls on this same issue and commonality between all the callers was they had updated to 5.1. I was transferred to 2nd and then 3rd level support who had me restore from new to see if that would fix and if not to call apple support. AT&T said they had close to enough calls on this issue that they would soon be escalating this to apple as a known issue with 5.1. Apple support 1st offered to send us new phones, but I told them that that didn't make sense since this only happened after the update and we would wait a few days to see if a new fix is made available. They couldn't find an official way for us to downgrade back to 5.01, but said I could search macroumors or google iclarify and try to find a way to downgrade back to 5.01. If you notice this on your phone, call AT&T and let them know, the more people who report it, the sooner it'll be addressed.