That's fine.
Just tell them that you've got other things to do and you'll get in touch with them as soon as possible or if you still have any transaction that will be done in the future.
Don't be too worried to end a conversation if you're saying an honest reason.
It really depends upon the situation. Sometimes we need to end the call early as we have something important to do, but for the person who is calling might need our time to solve any of his problem. I think if the other person is really in need of any help/advice, we should guide him completely before hanging up. However in case, he is just chit chatting, we can get an excuse from him and talk later.