Here is the top side of Jack's business card for M Station: Hawaii. Yes, he was still using his Hawaii Five-0 card, for the show was still in production when M Station: Hawaii was being planned and produced.
The telephone numbers on this card are very interesting. The top number, (808) 732-5577, was seen numerous times on the show. It was given on Kono's business card as the Five-0 number and in several episodes as the Moana (sometimes, Manoa) Cab Company number. The lower number links to Studio City in California. In 1973, it was Grant Tinker's telephone number at MTM Enterprises (Mary Tyler Moore Enterprises).
Then, as today, both were private telephone numbers. When the owner of 732-5577 (and probably other numbers) received many calls asking to speak to someone at a business mentioned on the show(s), the television suits realized they would have to find a way to use numbers that were not actual numbers. Thus was born the 555 prefix. Need a fictitious number for a story you're writing? Just let it start with 555, and you'll be safe.
PLEASE DO NOT TRY TO CALL EITHER NUMBER. BOTH ARE STILL ACTIVE TODAY.