For new CC the best way to approach this is to have a call volume lower barrier penalty.
This means for example that you need to work out that to stay in profit you need 100000 calls per month (based on a per call contract). You get the client to supply you with 3 months worth of volume forecasts. If the volume falls below their forcasted amount then you charge them for 90% of forecast per month.
If they under estimate their forecasts then you wont meet your SLA’s but you can say that you stffed according to forecast and so they would be to blame.
hope this helps