My home security system is one that I can call in to and arm, disarm, etc. With the original Iphone, this was simple. Call the home number, after one ring hang up and call a second time. On the second call the system answers the phone. I upgraded to the 3GS at roll-out and recently discovered that I cannot connect to the security system. When I call the second time the call rolls to voicemail. I noticed that calling home from my cell shows "Out of area" on caller ID with my cell's number displayed. Curiously, when 3G is turned off and I make the calls there's no issue connecting to the security system nor caller ID correctly displaying my name and number. I've confirmed with the phone company (Cox Cable) and the alarm company (ADT) that all is functioning properly on their ends. AT&T are suggesting incompatibility with 3G network and my phones and alarm system. When other 3G Iphone users call my home number their caller ID info is displayed correctly.....so.....I replaced the SIM card yesterday with no changes. AT&T have opened a "case" since they've no other reports of this behavior. Any thoughts/solutions would be most welcome!