is iphone 5 compatible with Panasonic KX-TG7743S cordless phone

No. I haven't tested this, but from what I can tell it won't work.

Panasonic has a list of supported cell phones here -> Headset ComplatibleList with Panasonic products

& Apple has a list of supported Bluetooth profiles here -> iOS: Supported Bluetooth profiles

If you want to give it a shot anyway, this might help ya out -> iOS: Third-party Bluetooth headsets, headphones, and keyboards

Yes, I was able to figure it out with the instructions here: https://discussions.apple.com/thread/4058360?start=0&tstart=0