zxcvbnmmlp, Is it mandatory to be a witness as a third party?

Hi, zxcvbnmmlp or the other expert,
I have a question. Is it mandatory to be a witness as a third party? My hu*****and and his ex-wife have a lawsuit on child support. If his ex-wife's attorney asked me or my friend to be a witness, can we object?If the court order us to be witness, is it mandatory to go? Thanks a lot!