>>To call people via a SIP address, it has to be put in the Fritz!box' 
> 'Has to be' John? Is that because you can't put the letters etc in the
> 'dial' string or simply because the telephone system wouldn't
> understand what to do with it?

Both.  You can't put letters in the dialstring, and an analogue phone can
not send them on an analogue line.

> Ok. I'm not sure exactly what choice I have re using the Fritz PABX or
> not if I want to use the single phone on PSTN or VoIP though?

If you enable 'direct outside dialling' (=no need to dial a 0 to get an
external line) on the analogue phone port on the Fritz!box, it seems like
there is no exchange at all.

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