Like @Brian Adams, I believe QBs is a good solution.  

The solution to this issue is to avoid it in the first place as best you can by screening and setting expectations on the front end.

If there aren't any good free solutions, is there a paid service that is recommended for real estate owners?  

Identify their problem/ needs, and show them why you are their best solution. 

I use a simple lockbox but you could also use  combination lock that had different codes for each guest that you can monitor and change codes Video WiFi.

For the calls coming in: the goal is to provide a solution for the customer.

These have their own searchable values that can only help the SEO ranking of your website.Additionally, you want to sign up for Google Analytics to track page usage with your visitors, & Google Webmaster tools to submit url's for crawl requests, monitor any webpage errors (301/302/403/404), & most importantly, just how your site is performing with Google SEO.  

Having said that, the number of possible solutions is relatively small and unless you have some unique set of circumstances (hence the need for a SME).

Essentially, there isn't an easy or inexpensive solution for sourcing phone numbers, and anybody offering a service to you for a low price has bad data. 

Any tips or solutions?