Deregistration

[buergstein]

I bought several T54W phones from private sellers.
Now it seems that these phones have not been deregistered because they connect to a "Starface" telephone system or another one.
Factory resets do not work.
How can I deregister the devices in order to use them with my telephone exchange?

Addition: I put the MAC of one telephone into the RPS-Removal-Procedure.... How long does it take? My new provider denies the registration of this MAC

Stefan.