In my neck of the woods, we offer maintenenace contacts based upon - mostly - a per port monthly charge - with or without set replacement - and another charge for the Voice Mail. Usually a factor of 10 ports per one voice mail port. In other words one voice mail port = 10 station/trunk ports. T1's are more.(Longer to resolve)
There are other variables such as 9-5 or 24/7 coverage.
Acts of God and Changes are never covered (Lightning, Power Fail, Surges, Change Password, etc.)
Telco related issues may not be covered. (Not our fault)
There are also formulas for 8% to 15% of the replacement cost of the equipment for an annual charge.
For Key Systems, it's expensive, but for a larger system, it's insurance against a major outage.
A company can also guarantee response times with a contract. (SLA for Service Level Agreement)
2 Hr for down system
4 hour for more than 10% outage
16 hour for routine service.
The rule of thumb is .... If something is sitting there working and stops working - it's covered. IF Homer throws the phone or spills his smoothie in there - it's not.
If you thought you'd like to program and screw it up - not covered, etc.
We use between $3 and $5 per station / trunk port. Less ports = more expensive per port.
A company can't make a lot of money on a few contracts. It can only make money on shared risk - and a lot of customers under contract....like real insurance.
YMMV, See Offer for Details, Offer Not Valid in Alaska and Tennessee, Must be 18 to Play.
Rick Williams "Doctor Dialtone"