Brian Gaff presented the following explanation :
I did this on Virgin, most operators offer this, however one issue seems to
be that some calls for no good reason seem not to always be included.
Too many different bloody number types around these days.
Brian

Get a notebook and print out which calls are and are not included in
the free calls. Every ISP has a list you can print out. Plusnet's
package includes all the common numbers as part of the package free for
1 hour, then 2000 minutes of free calls to mobiles. I'm not sure
whether the mobile calls have a 1 hour limit or not.