I have such fun with these calls.
So far I've ask them to hold the line as the bill payer will be with them in a minute and I amuse myself by shouting "are you coming, Norah?" Or other such nonsense just to keep them on the line. "She'll not be long. It's her leg playing her up, you know"
Another time it was "hello? Hello, speak up will you?". They couldn't hear me laughing they were shouting so loud.
Next time I might try pretending they've rung a sex chat line.