What are the specifics re: last furnishing and lien date?

Based on what small information is provided here, I do not believe that the service call extends the lien deadline for the contract work. You may have lien rights for the service work, but not for the original work in my opinion.

The 90-day period for recording a Claim of Lien in Michigan begins on the date of completion of the original installation work and is not extended by the later performance of warranty work. Warranty work is not considered an "improvement" because it does not confer any additional value to the property. See, Woodman v. Walter, 204 Mich. App. 68, 69-70; 514 N.W.2d 190 (1994).  

From the facts you presented, I think the "leak call" would be considered warranty work and would not extend your 90-day period.