You know, as Germans we've a DIN-Norm (standard) for everything :)
I have seen a variety of notations for phone numbers. Checked. And yes, we also have a standard - DIN 5008. DIN 5008 recommends us the following notations:
Phone number (including fax/mobile) with prefix: 030 123456
Phone number with area code and extension number: 030 123456-789
Special number (digit after the area code gives information about the rate): 0180 2 12345
Phone numbers with abroad (add dialing code with a leading "+", it eliminates the 0 prefix): +49 30 123456-789