I have tried to plug an extension telephone in an outlet in the house but the BB router flashes orange when doing so. The master socket has two outlets one of which is connected to the router
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The extension must be plugged into the phone outlet on that master socket.
Its possible your extension is not connected to the right place, which means you would have to put a microfilter in the lead to the phone.
Here is an example of what I mean.
Who provides your broadband and phone service?
Remove the master socket faceplate, extention should be dead. If not incorrectly wired. The extention cable should be connected to the back of the faceplate.