I just inherited a new location with a BCM50 that over the last week suddenly all the phones would say on the display no lines available. If a person is on the phone the call is dropped, they can't draw dial-tone and can't place calls internal. it will be in this state for an hourish and then just come back. I have no idea as to how this system was installed but with those symptoms is this most likely a PBX issue or a telco issue? The fact that they can't make internal calls makes me think PBX but the up and down sounds like it could be telco.