I don't have the phones you mentioned, but I was able to register by Binatone DECT handset with my new BT DECT base station.
You will need both manuals to complete the process.
1st you need to get the panasonic system setup and working okay with its 2 handsets. By default the panasonic base staion should be base station 1, if you just follow the phillips manaul for registering a handset it would not work because it will be looking for the phillips base station which would have been base station one (by default also but with a different base ID).
You now need to check the panasonic manual and see if there is a section that tells you how to find the base station ID, get this number, and there should be an entry in the phillips manual on registering a handset to a different base station.
If you still have problems let me know, and I'll post entries from my manuals when I get home, this might point you in the right direction.