Ask HN: stats on phone calls from website visitors?

Does anyone have any experience or notes to share on the effect on posting a phone number, such as a toll free number, on a website? I'm primarily thinking of sales-oriented websites for web-based products, not e-commerce or consulting/services.

I understand that it varies, but if you have anything to share I would appreciate it.

If you can't post exact numbers, percentages of total unique visitors or at least a ratio would be great.

edit: What I am asking is basically how many calls are generated from posting a phone number on a website, i.e. what percentage of website visitors who will call a phone number provided it is available.