Complex sentence: The exception is Helsinki: there is a telephone numbering area that comprises Greater Helsinki (code 09), while only the city of Helsinki proper comprises the electoral district of Helsinki, the rest of Greater Helsinki belonging to the Uusimaa electoral district.
Split into simpler sentences: The exception is Helsinki: there is a telephone numbering area that comprises Greater Helsinki (code 09), while only the city of Helsinki proper comprises the electoral district of Helsinki. The rest of Greater Helsinki belonging to the Uusimaa electoral district.