Dial pad doesn't allow numbers starting with '00'

The dial pad's '0' button has a '+' below the '0' but it works by translating a '00' prefix into '+'. Unfortunately some sip systems don't recognise these two as the same thing, which means I can't dial an international number from iOS because I can't enter the correct number.

This is also different from the system dial pad (which enters a '+' if you hold '0') which seems unnecessarily inconsistent.

We can't generally assume that '00' and '+' are equivalent: different countries use different exit codes.