iPhone cannot make/receive calls

[kr9ab]

My iPhone was bought from O2. It had 1.1.2 when I bought it. I restored to 1.1.4 and used ZiPhone to unlock etc. (I've since read about the problems!). Everything has been running fine for the past 4 weeks, with one exception. On the last three Sundays at about 1900 hrs, I get the "Repair needed. iPhone cannot make or receive calls. Contact Apple at www.apple.com/service." message appear. If I reboot it, works fine for a few minutes and then the message appears again. If I then turn it off and leave it overnight everything works fine again for the next week. I have had the (very) occasional lock up in the intervening periods, but nothing to really bother me. Any thoughts? I've got no cases with magnetic clips or anything else that could be considered a "constant", just the fact that it only (so far) happens on a Sunday!