I have had many listing agents contact me about either my preapproval or my company's policies and guidelines. I have never had anyone call me deceptively claiming to represent the buyer. I would question how they could...after all, my clients are all working closely with me, so I know who their agent is. I would never divulge anything about clients to anyone without explicit directive from my client. BTW, I don't think any mortgage professional would.
You can complain to the agent's broker (the manager who runs the office, the agent's boss) as a first step. You can also complain to the Department of State, they have a form you can complete.
Licensed Real Estate Salesperson