rendering bad service and not giving guarentee on work done

In November 2017 a technician working for SMR security replaced our intercom system at home. The technician required me to go with him and buy a new system on his companies name. he installed the system and repaired apparent damaged wiring on the system. The system only worked for about 1 month and is malfunctioning again. After contacting SMR security on 8 and 9 January, they refuse to repair the work. Their reasoning is that the technician no longer works for them and thus they do not guarantee the work.