Yes, it is okay for them to do so, though they should have informed you of the fee before answering your questions. You are using their time, knowledge, and expertise and there's nothing wrong with them wanting to be compensated for that.
My vet will primarily only answer questions over the phone that are related to an issue he is currently treating (so I've already been in with my pet for the current problem). He does not charge for this. Otherwise, if someone has a health concern with their pet, they are made an appointment to bring them in to be examined. It's a financial liability to give advice over the phone when they can't see and examine the animal and rely only on the owner's description of the issue.