1, 2 or 3 I'd expect, depends on the level of risk.
http://www.ciaalarms.co.uk/risk_assessment.htm
http://www.ciaalarms.co.uk/risk_assessment.htm
A call centre as in the alarm is 24/7 monitored by the central station and in the event of an alarm, they call the police and the key holders.It will depend on the grade of alarm, if it is one which connects to a call centre (who call out the police) then it will be a lot more sophisticated than a normal 8 zone alarm which may be fitted to a house.
A call centre? What grade is a simple domestic? No monitoring?