The real reason Skype isn't as good as it was

The real reason Skype isn't as good as it was The Register

... by inspecting the packets as they are transmitted through the network. Voice calls can be detected by their "signature" and when too many people are doing Skype or Vonage or Free World dialup calls, the ISP can set limits.
After (say) 100 sessions are started, "we can disconnect, or block new ones, or even slow down the rate they are transmitted," said Sandvine. "With customers who use this technology, we've got the traffic from 70 per cent to 20 per cent," he boasted.
One reason ISPs like doing this, is to encourage their customers to use the company's own VoIP system, rather than a rival's. If the packet is void, and is detected as from the cable company's own product, it is optimised - sent through faster than normal. Other packets are allowed to take their place in what's left of the queue.