Beeping heard during phone calls

Hi, all. I have noticed, in the last few months, a beep sound every so often while talking with someone on my phone. It sounds exactly like a beep you'd hear if you pressed one of the dialing buttons, but that's not the case. I almost always have the phone on speaker or on blutooth, so I'm not even touching the phone. The person on the other end of the line not only doesn't touch the phone, either, but doesn't even hear the beep. Has anyone else experienced this, or heard of any solution to it?

Thank in advance.