A polite way to say, "I will call you later" is:
"I'm sorry, but I'm currently occupied. Can I give you a call later?"
This response shows consideration for the other person's request while also indicating that you are busy at the moment. It leaves the conversation open-ended, suggesting that you'll reach out to them when you have the time to talk. Remember, being polite and respectful in your communication is essential in maintaining positive relationships with others.