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I've had Sony, Nokia, Moto and LG phones on AT&T/Cingular over the last 10 years and this has been a problem on all of them, along with "missed calls" showing up hours later. I've noticed it less with the iPhone, but it's always the important calls/voicemails that don't come thru properly, of course.
 
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_0_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5B108 Safari/525.20)

happened on my phone 3 times and I just paid my first bill.
 
I have had this problem for years with every phone, its an ATT thing for sure if you are using ATT

It happens on verizon as well with their normal phones. I have received notification of a vm 30 hours after someone called. Sometimes I didn't even get a missed call notification. It's with every carrier, sucks, but you have to live with it.
 
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