While we will dispense with discussions of Title III of the Omnibus Crime Act of 1968 regarding consent of parties to the call (per your statement of a benevolent dictatorship), the easiest way to record ALL incoming calls to a particular number is this;
Go to Radio shack and ask for a Remote Starter Switch and also get a tape recorder with both a microphone input jack and a remote input jack,
Plug it into any phone extension jack in the house that YOU have sole control over. Directions are on the RSS package.
If you don't have an extension jack that has limited access (that she can't get to) trace your phone wires in the attic (if available) and install a phone jack (in the attic, if necessary) and install the RSS + recorder.
This application is easily identifiable by disconnecting ALL phone in the house and then testing the lines with a good volt meter using the ohms resistance setting. With no phones attached to the lines, all lines should read "open", but with a RSS left attached, the line should read almost infinite resistance (in case you suspect someone has done this to YOU without your permission.
We won't argue the legalities of one-party consent (federal statute) or all-party consent (some states statutes, ie Maryland, Illinois, etc) in this discussion.