Can I add two handsets to a existing system of phones if they have different model numbers for the base units one is kxtg7643 and I want to add them to kxtg6641 or vice versa

SOURCE: Uniden DCT648 register phone problem

On the Base Station select HS Registration then on the Handset hold # for 2 sec

SOURCE: sync a new handset to kx-tg5632 phone

This works on MY panasonic; May work on yours too!

Registering a handset
The included handset and base unit are
preregistered. If for some reason the
handset is not registered to the base
unit, register the handset.
1 Handset:
{MENU} i {0}{0}{1}
2 Base unit:
Press and hold {LOCATOR/
INTERCOM} until the IN USE
indicator flashes.
LAfter the IN USE indicator starts
flashing, the rest of the procedure
must be completed within 1
minute.
3 Handset:
Press {OK}, then wait until a beep
sounds.
4 {OFF}
Registering an additional handset
See page 5 for information on the
available model.
Start from step 2.

SOURCE: how do I add an extension handset to my current config?

IF it was a used handset it was likely registered to another base. I just switched bases (1line to a 2 line base). I had to "deregister" the handsets from the old base. I am not sure if you can deregister if the old base is not present? With the old base plugged into power I hit the "function key" at the bottom left of the handset and scrolled down to initial settings. Under registration is "deregistration". I selected that and it put the handset back to its default out of the box. Then simply hit the mute button on the New base and OK on the handset. Worked for all 5 of my old handsets.

Hope this helps

SOURCE: i want to add an additional handset to my vtech phone...the new handset is "searching for base" and requests a 15-digit number which is suppose to be on the base--there is no 15-digit number to input.

The 15 digit number shoudl be on a sticker on the bottom of the base unit.

SOURCE: Unable to talk on two handsets at same time when receiving a call

Unfortunately that is the untold story about cordless phones they only have the capability of 1 open line be it the handset or or the base. There are some very pricey multi line systems, I have the same system bought two extra handsets and found out too late that I can only use them one at a time! Sorry probably not the answer you were looking for but thats the reality...