Maybe you can give us some more clues:
[ol 1]
[li]When a set is ringing, there is a message on the display. What is this message? Is it the same on the other set?[/li]
[li]Also, when a set is ringing, there is an indicator flashing. If you press Feature * 0, then the button next to the flashing indicator, what appears on the display? Is it the same on the other set?[/li]
[/ol]