I assume you have a contract, this should have written in it the maximum time period for deposit returns. If it has gone over this time I'd suggest writing them a letter as they clearly do not seem to want to deal with you professionally over the phone. The letter might write something along the lines of giving them a specific period (say, 7 days) in which to return your full deposit as they are contravening the terms of the contract (quote the exact paragraph if necessary). If this demand is not adhered to then legal action shall take place (small claims court) and all costs incurred by yourself shall be forwarded to them. Remind them also that they rely on students for their business and word of mouth is a very strong marketing tool.
Thank them for their cooperation and say that any communication from hence forth must be in written form and that you are taking this matter very seriously.
May seem a little extreme but often just a threat might be enough to get them to pay attention to your case, which should help you recover your deposit.
A quick note though, if it has not reached the end of the time specified in your contract for the return of your deposit you are probably helpless. My limit for my last house was 45 days after the end of my contract and the deposit finally reached me on the 43rd!