Just bought 7645 (earlier today) and was able to link an iphone 4s and motorola bionic. I had the cell phone near the base. On the panasonic base handset, touch Menu (middle softkey) then # and a 4 digit number - for cell 1 it was 6241. While the panasonic handset displayed registering, I went to the cell phone bluetooth function and had it find the device. When asked for the code (on the cell phone), I entered the PIN. from the panasonic. The default PIN is 0000 unless you changed it on the base firstonly a couple of calls. I had the cell phone bluetooth find the panasonic unit. did the iphone as cell 1 and the droid as cell 2.
I got it to work, but to the receiving listener the voice quality is terrible. I left myself a voice mail at work by calling out using the cell phone line and yes, the voice quality is terrible with the iPhone 4S iOS 5. I am not sure how to fix this problem. Regarding connecting, follow the instructions and then on your iPhone go to Settings >> General >> Bluetooth. Make sure bluetooth is "on" of course, then when it displays the model number of the Panasonic phone, click on that box on your iPhone, then it will change the screen and show it's attempting to link to the base unit (which it does -- it just doesn't work very well).
UPDATE 10/21/12: I upgraded to iOS 6 to see if that would fix the problem, and it did. When I speak using the Bluetooth option it now sounds fine.
I have IOS 6.0 on my 4S and still had a problem with badly garbled audio. I think that I have finally solved the mystery - at least mine. I use an older Linksys router for my local net. It transmits at 2.4GHZ and Bluetooth is close to that. Since the Bluetooth signal is nowhere near as powerful as the router, I was apparently experiencing interference. My signal is great if I turn off the WiFi connection on the phone OR if I make sure that the iPhone is always within two feet of the Panasonic base. Incidently, I was able to pair the iPhone 4S and my wifes older Samsung with no problem