We are having the same problem when someone calls into our Panasonic phone system and tries to be transferred to an extension. At first I thought that it was only when someone called in when using an iPhone and AT&T M-Cell because we have to have those here and cannot call into our office from our iPhones. Then our owner called in from his home, but he also has an M-cell. But just today one of our salespeople called in and said he could not enter in an extension.

I then tested it by calling in with Skype and was able to enter the extension and get through.

Then I tried calling in with the iPhone Phone app and using the Skype dialer for the tones and that did not work (it heard the tones, they just were too long).

It could be AT&T's service that is causing the long DTMF tones. I know that the tones sound longer on the other end than they do from the iPhone because I can tap in and listen to the dialing when someone calls and enters an extension. So I dialed in from my iPhone and I could hear the tones when I pressed them on the phone and what they sounded like through the system. The tones thru the system were much longer than what I heard from the iPhone.

So someone along the way is lengthening the tones. I'm guessing it is AT&T and not the iPHone. But that is just a guess.