What constitutes a "fair" VoIP SLA

[voipadmin]

A "fair" VoIP SLA ensures that the customer will get acceptable quality service but avoids the service provider having to meet unrealistic performance objectives. Factors in an SLA can range from

Per call or per metric threshold?
Should SLA's be defined based on MOS score (which includes everything) or on individual metrics?

Measurement period?
Percentage of good calls/tests per (24 hrs, 7 days, month..?)

What to include?
E.g. should noise level be included (which could be environmental and not due to the service)

Active or passive measurement?