I'm female and used to work in a department that processed spare part orders for our company's Field Service Engineers across the country. They would call in their orders and we would process them. I got on really well with the majority of them and there was a lot of banter back and forth.

One of my favorite engineers would leave a message for me to call him back if I was busy when he called. Sometimes it would be at the customer's location and sometimes at his home (no mobile phones then). He would often answer the phone anticipating it was me with some funny/corny greeting and we'd have a laugh.

One day I got a message to call him at home. I don't know what came over me but as I was dialing, I suddenly decided that today would be the day I turned the tables on him!! As soon as the ringtone stopped, I started heavy breathing down the phone. There was silence on the other end and I just kept going until I heard "I'm sorry, my husband isn't in right now"!!!!

TL;DR Thought I was telephone pranking a colleague but pranked his wife instead.