at (one's) service
Available to help or serve one. Here I am, resident file clerk at your service! How can help with this project?
at someone's service
Fig. ready to help someone in any way. The count greeted me warmly and said, "Welcome to my home. Just let me know what you need. I'm at your service." The desk clerk said, "Good morning, madam. I'm at your service."
at (someone's) service
Ready to help or be of use.