You can tell that your policy is that you'll only consider a written proposition, you won't discuss it on the phone - end of the conversation.
Make it short, be kind and polite, just make the other part fine with it, but remember the policy.
A friend of mine told me his method : " in NO, which part do you not understand ? " - funny, but I thinks it's too rude.
Personnaly I often orient the conversation about my products with a lot of enthousiasm - maybe I could sell something to the person who calls (then I don't make it short). Afterall, this phone line is here for your customers, then use it in this way.