Brussels is 1.5 hours behind Tehran

Best overlap: 9:30 AM to 6:30 PM in Tehran is 8:00 AM to 5:00 PM in Brussels.

For the smoothest call, aim inside 9:30 AM to 6:30 PM in Tehran, i.e. 8:00 AM to 5:00 PM in Brussels. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. Daylight saving complicates things slightly: Brussels observes it, Tehran doesn't, so the difference flexes by an hour part of the year.