Corrected entry: When all three men are together, writing notes to Foster they are all downstairs, you can see the front door in the background. Instead of carrying on with the conversation she could've just run to the bedroom and called 911 on her cell or the home phone.
Correction: At this point, she could not be sure there were only three men in the house. She was unfamilar with the surveillance system, as well as being scared and confused, so it makes sense that she was not thinking clearly.
While the facts in this correction are accurate, Meg eventually left the room to grab her phone without still knowing if more than three men were actually in the house, so this correction is a bit iffy.
Answer: Meg was using the P.A. from inside the panic room and it only works one way; everywhere else would have a room-to-room intercom system. If there was another P.A. system working the other way, it would have been too loud, as the cop mentioned when he spoke to Meg at the front door. The neighbors heard Meg broadcasting on it. Burnham would know that it would draw unwanted attention and, if heard, could be incriminating.
raywest ★
Cop didn't say anything to Meg about neighbors hearing her on the PA. They were only there cause the husband called the cops after she was disconnected from him.
The cops said to Meg that neighbors called in about a loud disturbance, so they did hear the PA system. Meg covered those tracks by saying it was the TV.
There are two possible reasons that I can think of 1) Burnham probably simply forgot that he could do it, the same as none of them thought to smash the cameras until Raoul saw Meg doing it. They were under a lot of pressure as the plan had changed so it probably just slipped his mind. 2) Burnham knew he could do it but didn't want the other two to know. The other two seemed like loose cannons and Burnham seemed to be the only one who didn't want to hurt anyone. He might have been worried that they'd use the intercom to send threats or something.
It was a one-way PA system, as mentioned by the original answer.
No, it's not. After the burglars get into the room with Meg's daughter, Meg uses the intercom in the bedroom to talk to Burnham who is inside the room. This indicates that it is very much an intercom and not a one-way PA system.