Howdy.
As you said, it will depend on the industry. But not only. It will depend on type of service/product you offer. Example: direct purchase of simple thing like a tshirt or mug or whatever, when people wouldn't have any questions, phone conversion will be really low, since the only people calling will be somewhat unqualified buyers.
Here is how we do it. So, on a landing page we have landing page specific phone number with tracking (use Twilio). And all sales are tracked in the admin. So, if someone calls, using this phone number, and makes a purchase, we have a logged time of that call and time of the purchase, placed by representative. All we have to do is match it. You can do it manually or with some script.
In terms of ballpark of conversion rate, it's typically 30% from landing page. Depending on the industry of course. So, that's average rate. Therefore if you know rate for the form, you'll be able to calculate avg conversion rate for calls.
Hope this makes sense 