Thinking about you given scenario a little more, I think you could plug the RTX base unit (currently plugged into the BT master socket, via a splitter) straight into the FON port on the back of the Fritz!box and then using the same splitter plug the other analogue phone into the splitter also.
This would enable you to re use all your existing hardware / phones with minimal changes etc.
Alas I am not aware of any attachments you can add to your existing analogue phones that will support DECT connectivity directly to the Fritz!Box.
would be a interesting exercise 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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