strange bleeping noises when calling a droid

[angelman]

My wife has a droid and when I call her from my office phone I get strange electronic bleeping sounds every few minutes. There is usually one about 30 seconds into the conversation and then every minute or so.
They are quite high pitched and loud. By bleep I mean it's like 3 or more high pitched tones in quick succession, almost like an old school modem sound..
I only hear it when calling from my office phone. Should she take the phone back, could there be some weird interference between my office phone and the droid. Hers is the only phone it happens to. I havent tried it with my droid yet