Welcome to this party. Ugh. Been there. I went through SIX iPhone4s. The first 4 did this same thing, where it would work for a while, then start searching constantly for a signal. When it did have one, it would not make outbound calls without not being able to connect or hear a connection on my end. I had 8 sim cards by then also, I pretty much went between ATT and Apple stores bec both blamed the other party. Before they gave me the fifth one, Apple got upper level tech involved at ATT, and they reset all kind of stuff on my account. Suddenly I was able to keep a signal, but I was not able to make outbound calls properly yet. I JUST got the 6th phone AND my 9th Sim about two weeks ago and this is finally the first time with a 4S ive been able to make calls AND not be searching all the time. There is something wrong between the SIM/Account/Line and the phone. Call Applecare and get a Senior Advisor, and then they can get ATT on the line and work with you. It will likely NOT be a quick fix, jsut be aware. It took over 2 months to finally get a functional situation.