Flat rate with unlimited minutes just doesn't make sense. With per-incident pricing, the outsourcers tend to try to cut these calls short by rushing things along. The more incidents a representative can handle in an hour, the more revenue they can create. The customers may leave the call feeling unwanted or upset. With per-minute pricing, the opposite is true.
Representatives become chatty and drag the call out.
A higher level of customer satisfaction may be achieved, though. Per-subscriber seems to be the safest scenario. There is no incentive to rush calls along or to draw them out. This pricing seems to force the right level of support. But a limit is necessary.