All devices must be properly connected to function properly. Therefore, make sure that the telephone cable is connected to the correct socket of the modem, then to the subscriber socket and, in the case of a cordless telephone, its power supply is also connected to the electric socket. For verification, see the Simplified Diagram of connection.
If you use a so-called parallel line at home (ie several telephones connected to the telephone socket of our modem at the same time, when both ring at the same time when there is an incoming call), there may be a fault on the hub. Before starting troubleshooting, connect only one device to our modem according to the diagram.
Recommendation: At the beginning of diagnosing the cause of the problem, always restart the modem by disconnecting it from the electric socket for a few seconds and check the status again a few minutes later (after you reconnect it again). If the call does not work correctly, the registration of the telephone may have been interrupted.