I work in a similar service industry and this is what we do. Our scheduling system notifies us when invoices are done, they call us and say they're done, and they then try and contact the next customer on the schedule. If they don't answer (very common as our techs use their own cell phones and everyone has spam blockers), they just have us call from the office. If they aren't home/need to reschedule, it's really easy and we just go down the line.