Or, perhaps the tower called the company and told them to tell the pilots to give them a call. They may well have been in the blocks when they asked for the tower phone number on the ground control frequency.
Seems a bit unlikely... if the tower called the company presumably the caller would've left his/her name and phone number for the callback.
I can imagine the conversation... "AC please ask the Captain to call me over the radio to ask me for my phone number". When the controller could've just radioed the pilots directly.