of service (to someone or something)
Helping, serving, assisting, or working for someone or something Good evening, sir, how may I be of service? He has undoubtedly committed a transgression, but in consideration of his many unblemished years of service to the force, we are recommending unpaid suspension rather than complete dismissal.
of service (to someone)
helping someone; serving someone. Good morning, madam. May I be of service to you? Welcome to the Warwick Hotel. May I be of service?