Consider this:
Many salespeople conduct a large part of their negotiations (perhaps 50 percent or more) over the phone. If a customer/prospect calls you to wrap up a deal, do you suppose that she has prepared for the telephone conversation? You betcha! She's gathered all the information she needs to support her position and she's 'ready to launch' with the perfect rationale for making the deal on her terms.
But, have you done the preparation necessary to negotiate? Have YOU gathered the information you need (let alone your thoughts) to receive the phone call? Probably not.
Resist the temptation to jump in and hash it out without proper preparation.
Find a reason why you can't talk right now; then set an appointment to return the call (even if it's only 15 minutes later). Use that time to gather your information and plan your strategy.
You'll find that the outcome is much more to your satisfaction!