Why did you ask this from the lender's side of things i.e. "I have a lender tied with my agent..." First I would investigate and make sure everything is on the up and up. For instance your listing agent has a REALTOR's license and so does the Buyer's Agent, otherwise they may not be ethically bound to work together or licensed.
If everything is on the up and up; your agent should have his/her broker contact the listing broker and get on the same page but before that your agent should have already gone through the steps of writing up an offer, and provided to the listing agent. This Real Estate 101 and your agent for sake of this writing submitted the paperwork then there should be a response, also in writing. Again if not, the Broker's should be informed of the non-compliance. My feeling is that someone is not following process or something weird is going on. From experience I have had this happen when a listing agent was holding off a property for another buyer, fearing my offer would get accepted over their friends. That is when the brokers step in. But again, your agent should know all this, it is basic stuff and makes me wonder if it is your agent and not the listor.
I saw your question. If the listing agent does not respond to your agent phone calls or faxes, your agent could talk directly to the Broker of that listing agent & say "I was unable to receive a response from XXX so I am contacting you for ...". You could also obtain the information of the seller but remember you have an agent that represents You and will be looking for your best interest so let her/him do his/her job. Good luck.