Will I get a better deal if I call the listing agent to buy a house?

Overall, no...
The listing agent first and foremost represents the seller and their best interests.

It's usually best to work with your own agent, one who will do what's best for you. Work with an agent you have a rapport with, someone you trust, and that isn’t afraid to ask questions. An experienced agent is a skilled negotiator and ultimately they are working for you.