As a Central Office technician for a major IXC/CLEC, I have noticed that larger companies have been migrating voice service to VOIP.
Such companies, that can afford to purchase VOIP through a managed service provider where you tie directly into routers via a dedicated T1 or T3, usually fare well.
The problem is felt more by small outfits, like mom & pop stores, churches and such that are not high volume callers and cannot afford loop charges. These groups, if they want to use VOIP, have to rely on providers like Vonage or Packet 8 and connect to these providers via regular DSL or high-speed cable access. The QoS provided by companies like Vonage and Packet 8 has improved over the years, but cannot match the QoS of an end-to-end managed service provider.