When I received the new system I did try to register one of the old handsets and it did not work. I could see no physical difference between the handsets except the new ones have a green color around the display. The batteries and chargers are identical, as is the connector for the external antenna.
Sounds like you where tring to registor a "Pro" durafon to a 4X base unit.
Keep in mind that just any of the Durafon handsets will not work with just any Durafon base unit, not even 1X and 4X vs Pro.
In short, if you have a 1x handset, you must have a 1x base unit, if you have a 4x handset, you must have a 4x base unit and if you have a "pro (the one with the green label)" you must have a "pro" base unit.
