@Dunstan1
In my experience as a live-in host for 6 years I hardly consider this an issue of any sort.
Firstly, a good WiFi means guests can reliably and freely use their mobiles, and have a variety of means of making free calls WhattsApp, Facebook, etc.
So its very rare that a Guest even asks to use the landline, and whoever has offered to pay; and I can hardly recall accepting their generous offer for pennies or whatever the charge which is negligible.
Perhaps as an absentee host you have greater concern. But I probably wouldn't take special measures.
I would rather write it up in HOUSE RULES politely asking Guests either not to use landline or if they need to with your permission, alternatively to keep a record of their calls. In any case, records will be in your bill.
Thus the only conceivable problem is if Guest flouts your House Rule and calls without informing you; but you will still have record on your phone bill and can send a claim via Airbnb. It will not be disputed because you will not have been in the property, so it must obviously be the Guest.
You can further back this with listing a Deposit.
Payment can be made via Airbnb payment or Resolution Centre.
Phone bills are monthly, though you can still check in between time on line for current use of phone.
Airbnb claims can be made up to 60 Days post Guest departure: [See 'Send or Request Money' in Inbox of a booked Guest].