Dialing sound during conversation on iPhone 4S/5

I noticed a strange phenomena during the past several months (probably 4 or 5). I originally had an iPhone 4S that presented the weird problem, but I just had it happen today on my new iPhone 5 (which was set up from a back up of my old iPhone 4S). I have noticed when using the phone that I will hear a dialing noise as if the person I'm speaking to has accidentally pressed a number pad on their iphone. My parents have an AT&T iPhone 4 and it was happening with such regularity during one conversation that I was convinced my mom was hitting the buttons on the touch screen with her cheek. Then I noticed the same pressed button noise in other calls as well. It's just a momentary noise like a button to dial a phone number had been pressed. It's not a long tone. It's not a multi tone, but it is really annoying. I was looking forward to upgrading because this was starting to get on my nerves, but now I was upset to have the same noise present on my new iPhone 5 this evening during a phone call to my mom. It's happened when I've used the phone connected with bluetooth in my car and also when I've used the speakerphone at home. I almost never put the phone up to my head - 99% of the time, I'm either on bluetooth or speaker. Anybody have any experience with this or have any idea what could be causing this. The caller never hears the dialing noise either. I've asked both my mom and my wife and when I've heard it during a conversation with them, they've never heard it on their end.