It happens that Philippe Deleye formulated :

>>
> A few more tips to get this correctly done:
> * register your landlines as descrobed above, but make sure you use the
> format 0044 xxxx, not in the format +44 xxx
> The latter will work for verification, but not for CLI..
> * set your SIP device or IP phone to use your phone number, and also use the
> same format (0044 xxxx) here.
> If you have dones this correctly, it will work perfectly, even for non-UK
> numbers.
>
> Philippe.

I will try my German number in a bit, however, even though I have set
everything up as 004401 or 0044870 (for the UK ones), the number that
is presented is not in the format 00441, nor +441 but 01. Therefore, I
suspect that the German one would be correct for dialling a German
number......