Is this for a government agency or private company? Private company if it is small, you can call right before you are sending the response to the RFP - say you want to verify whose attention you should send it to (or some piece of information that is not clearly on the RFP) and try to get into a conversation with the person responsible for collecting the responses (this person might be the decision maker, or maybe not) - try fishing for information - are they getting a lot of responses, casually ask what they are basing their decision on, etc. Your conversation may reveal something that will make you want to change something in your proposal. Also, you may have an opportunity to introduce yourself and your service through a call and then stand out when your proposal is viewed. It kind of depends on the situation - some RFP are just inappropriate to call on (like if it says "no calls please") so you have to kind of play it by ear.
I've also had luck winning bids by adding personal touches to my services and then mentioning that on the phone when I called.
I've also had luck winning bids by adding personal touches to my services and then mentioning that on the phone when I called.