When you are working with a buyer's agent it is important to remember that your agent wants to help you purchase the home that is right for you. It is also important to remember that they won't be able be with you at all times. If you are driving around on a Sunday afternoon (or any other day) and you happen to see an open house, it is completely fine to stop and check the place out. who knows it may be outside of the area where you want to be, and it might not have all of the features that you are looking for, but you may fall in love with it. It is important for you to mention that you are working with an agent already to the agent that is holding the home open. Most states have specific rules in place for Realtors that define when one agent can represent a buyer, and when they can not. If you begin negotiations with that agent your buyer's agent may not be able to step in a represent your best interests. In that situation you may lose all of the experience that your agent has gained in their career, and negotiate the terms on your own. Your agent has specific market knowledge and tools available to them, that can't be learned overnight. It is always in your best interest to have someone working on your behalf. So always just remember to mention them when at an open house.
If you are under contract with a buyer's agent it actually may be in your buyer's agent contract, which is just a formal agreement stating that the buyer's agent is going to do everything in their power to find you the best possible home that they can, and you in turn agree that should you buy a home, it will be through that agent. You might want to look at your contract if there is one in place, and see if it specifically says anything about signing in at an open house. Each contract will be different so there is no way to tell if your specific contract includes it without looking at it.
Another thing that you can do is to write your agents name and number on the sign in sheet, this will enable the open house agent to get in touch with your buyer's agent, and stop them from calling your phone directly. Working with a buyer's agent has lots of advantages, and stopping unwanted phone calls is just a bonus!