What's that beeping?

Tiffy, after receiving two calls and hanging up quickly: "What is it this morning? Is someone playing with the phones?"

Me: "What is it - a fax machine? I've had a couple of those already." 

Tiffy: "No, it's beeping." 

Me: "Fax machines beep." 

Tiffy: "Fax machines don't beep. Fax machines go, "eeeeeeeeee," like that."