If you want to track your off-site SEO with a virtual phone number, your Google Places (as it is increasingly important), will be hurt in rankings because the phone number will now be inconsistent with the on-page # as well as any pre-established directories (pre-established, meaning before you took on the project). If you want to keep the virtual number both On-site and Off-site, then the "value-added" proposition is diminished since, if the client already had a website and a phone number, there would be no way to tell the two apart. Does this make sense?
Quite a lot of sense. But much have happened since this was written, that gives a more better approach than this.