Dear Ms. Reid:
I have contacted a (very) small press about my novel and we have gotten to the point where they have requested a full. In the event they accept this manuscript, would it then be all right for me to telephone an agent (maybe even you, perhaps) to negotiate the contract?
no no no.
In this circumstance you will EMAIL an agent to inquire if they are interested in helping you negotiate the contract. (a good example can be found in this BookEndsLLC blog post)
And probably you're not going to find one who will because 15% of a small advance is usually not enough money to make it worth while. Most agents or agencies have standard contracts with publishers with whom they do business on a regular basis. We don't have to renegotiate most of the terms and conditions on each deal.
With a new publisher and a small press, much of that has to be negotiated and it's time consuming as hell.
If you get to this point and you can't find an agent to help you, email me again and I'll give you the name of a contract review specialist who charges by the hour and can give you advice on which clauses you can/should negotiate with the publisher.