What is the best way to control the agent's adherence in the call center ?

What we can do here is that let them work in pairs. So they can go for their breaks in pair & they would return back faster. If they work in pair they remaining absent also reduces as the other guy has to compensate the work. Break times should be monitored. Introduction of Cash Incentive will do the needful.

The Team Leader/Floor Supervisor needs to gel around with the agents, at the same time monitor their adherence.Give a strong feedback when required and inform them about the KPI's / Appraisal and most of all an incentive plan for adherence.

The best way would be managing his/her time. On time log in to start the shift and of course break time should be monitored. Also, if there is such a workflow to follow, it should be strictly complied with.