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Remove the plastic battery cover from the handset (but keep the a battery connected), then place the handset on the base. That will enable the metal contacts at the bottom of the handset to touch the ones on the bottom of the base.
If the handsets are not currently registered to any base, the SYSTEM RESET menu won't appear.
If the TCX905 handsets in question are actually expansion handsets from another Uniden Phone System, they are not true TCX905 handsets. To verify, remove the battery cover and battery, then look at the inside label. If it lists the model of a TRU9xxx phone system, your handsets are NOT TCX905.
Uniden unfortunately has programmed the expansion handsets from SINGLE LINE TRU9xxx phone systems to NOT REGISTER against the TRU9466. (Those same handsets register fine against predecessor model TRU8866 though).
In order to re-register handsets with TRU8866 base unit, you have to reset the base unit, de-register the handsets, and then re-register the handsets. To reset the base station, disconnect the power from the base and then press and hold the CID, Intercom, and Exit keys while you connect the power. Wait for the beep, and let go of the keys. Now re-register the handsets - the first one will become #1, and so on.
The "END" and "#" key combination can be used for several purposes:
1) If wanting to use an ALREADY REGISTERED HANDSET on an alternate base, and if if the INITIAL BASE isn't available to deregister the handset:
1a) Press "END" + "#" until it for 5+ seconds (wait for it to beep)
1b) Choose the "Base Unavailable" option
1c) That'll deregister the CURRENT HANDSET so that it can be re-registered on ANY base.
2) If a BASE has a handset registered to it which you no longer want registered but the registered handset is missing or broken:
2a) Grab another WORKING handset which is properly REGISTERED to the BASE in question
2b) Press and hold the "END" + "#" key for 5+ seconds (wait for it to beep)
2c) Choose the "Deregister HS" menu option
2d) Pick the REGISTERED HANDSET # to de-register. (Every other registered handset EXCEPT THE ONE YOU'RE ON will appear in the list!)
1. Disconnect Battery from handset(s)
2. Disconnect Phone Cord from wall jack
3. Disconnect Phone cord from base unit
4. Disconnect AC adapter from wall outlet
5. Allow to sit for 30 min
6. Reconnect Phone Cord to base
7. Reconnect Phone cord to wall jack
8. Reconnect AC Adapter to wall outlet
9. Reconnect Battery to hanset until you click or feel the plug snap into place
10. Re-Register Handset by placing on main base in most instances.
This exact thing happened to me. Same model number. No matter what I did, the new handsets would not register.
The solution? Reset the entire system: 1. On the BASE - Unplug from wall both power and phone line. Unplug battery from base handset. 2. ON the Extension phone - unplug from wall and unplug battery from handset. 3. Wait AT LEAST 5 minutes. 4. Plug back in BASE power. 5. Plug back in BASE phone line. 6. Plug back in EXTENSION handset battery. Don't plug in BASE handset yet. 7. Follow steps to program EXTENSION. When successful: 8. Plug back in BASE handset battery.
Same manual as lethean provided on a similar post but Page 24 if the previously used Base is Unavailable and your phone shows "Deregistration" in the Menu.
Base Unavailable (Handset Only)
To clear the base registration data in the handset, please follow these steps:
Press and hold  and [#] for more than 5 seconds. Select the BASE UNAVAILABLE submenu.
BASE UNAVAILABLE appears. Move the cursor to select YES, and then press the OK soft key.
ARE YOU SURE? appears. Move the cursor to select YES. Press the OK soft key.