Have had ishoos in the past with the PILs overstepping boundaries a little (DH died so I can't go through him in dealing with them). I may well be oversensitive now because of this.
Anyway, if you'd gone out and your PILs were at your house babysitting, would you mind or not if they answered your landline phone when it rang?
Bearing in mind that I have caller display and an answerphone so they would know that it wasn't me calling and that whoever did ring could leave a message. And they have mobiles which I could ring anyway if needs be.