Shirley has a Samsung Galaxy S7 with a Motorola Bluetooth earpiece. With the recent March update, the connection switches to speaker phone automatically for no reason. Neither Samsung or Motorola know what it is. Leo says that he's tempted to blame Bluetooth because it's kind a dark art. It could also be a setting in Nougat that causes it as well. There is an issue tracker page for bug reporters in Android for wireless settings at issuetracker.google.com.
Leo says to try forgetting the connection and reacquiring it. That could solve the issue. She should also look in the "Backup and Reset" settings in Android and reset her network settings. Many have found that helps by eliminating previous connections that relied on the old version of the Android OS.
Doctor Mom in the chatroom suggests restarting the phone. Turn it all the way off, then wait a few seconds and turn it back on. That will reload everything. Many suggest a double restart in safe mode can fix it too. There may also be a fix that Samsung hasn't applied yet.