FIRST STEP - to "protect" the old system components from changes.
1) Disconnect the batteries from ALL of the OLDER handsets.
2) Disconnect the power (and phone cord) from the OLDER Main Base Unit.
LEAVE THESE COMPONENTS DISCONNECTED during SECOND STEP (below).
SECOND STEP: DEREGISTER NEW handset from NEW base.
1) Use the "To deregister all handsets" instructions I just sent.
Once the NEW handset shows the "To Register HS..." and "... see manual" (alternating), move on to THIRD STEP (below),
2) Disconnect POWER from the NEW base.
THIRD STEP: Register NEW handset to OLDER BASE.
1) Reconnect batteries in OLDER handsets.
2) Reconnect phone cord and power to the OLDER BASE.
3) Follow the steps to Register the NEW handset to the OLDER BASE.
I believe you have these instructions already, as I sent them before and you mentioned that you already had them. But let me know if you need them again.
Hopefully, this will register the NEW handset to the OLDER base.
If that works, then it should be OK to reconnect the power to the NEW base - although it will now be used ONLY as a "Charger".
I hope that all makes sense and can be done.
Please let me know if you have any questions about this process.
Thanks, ***** *****