Make it part of the process, send a push notification once their issue has been fixed and ask them to take a photograph of the fixed tree/rubbish free space etc to "verify that the work was done".
The app that Detroit was built on, SeeClickFix, came years before the NYC app and has the added benefit of displaying all other existing open issues. Detroit and SeeClickFix also support the Federal Open Data Standard, Open311 which is groundbreaking in itself.
smartphones have proven to be the number one technology bridging the digital divide in the US. If you call into the City of Detroit it will also be publicly documented in the same system: SeeClickFix.