The main issue seems to be that their support staff are saying they will call back, when they simply won't. From a customer journey/CRM perspective this is poor, and would be easily solvable. Either stop saying to customers they'll call them back or ensure that all callbacks are completed by close down of service every day.
A very simple and inexpensive attention to detail measure.
Andrews & Arnold Home ::1 on Draytek 2862ac - Why settle for inferior?