Someone call me a cab

Q: Someone call me a cab.
A: Your a cab.

This is a joke that requires the person that is responding to the question to be in on the joke (or the joke has to be said by the responder without the foreknowledge of the person that initiated the joke).

The humor is a pun - the joke initiator tells the second person to "call" them a cab, which could mean for the second person to call a cab for the first person, but the second person parses the sentence as call me the name 'a cab' (instead of calling them whatever name they were calling them before).

The second person is likely deliberately misunderstanding the first person in this joke, because it is highly unlikely that a person would misinterpret the sentence in that way.