quoting Debian bug #440159:

Put keyboard focus in the "what sip address shall I call" text entry
box. Start entering something. Put cursor anywhere else than end of
the string currently contained in the text entry box, such as
beginning of the string or somewhere in the middle. Type digits with
the numeric keypad of your keyboard.

The digits get added to the _end_ of the string instead of at the
cursor's position. Strangely, the digit row above the alphabetic keys
works well, the digits get inserted at the cursor position.