Set up a Fritzbox 7530 for IP telephony

Context

I have a Fritzbox 7530 with a static IP address, but all of this applies also to dynamic IP.

First steps

1. In the first instance don't buy a DECT phone, instead install "FRITZ!App Fon" on your mobile.
2. Log in to your router
3. Select Home Network > Network Settings
4. Ensure that "Allow access for applications" is enabled.
5. Open the FritzApp on your mobile and follow the prompts to set up everything you need. It will log you in to the router, configure the telephony settings, and restart the router.
6. Add as many other mobile handsets as you wish. Each will have an internal phone number for direct dialling between handsets.

You will now be able to receive "landline" call wherever you have an internet connection, not just in your own home. I have not found a way of making calls from outside the range of the router.

Afterwards

If you really want a dedicated handset then I am informed that Gigaset CL660HX works with this router as should alll of their HX series. Also Fritz!Fon M2, C4, C5, C6 and X6; and Panasonic KX-TGJ3xx series, KX-TGD3xx series, KX-TGK3xx series, KX-TGQ500/400/200 are all confirmed to work with this router. Note that I have not tried them myself..