This is a suficiently grey area for the CAA not to get worked up about. After all, airline crewing departments will produce rosters that are not in the spirit of CAP371, but "not in the spirit" is a long way from illegal.
If this was a rare occurance then the airline were just doing what they could to get out of a sticky situation. If this is happening alot then it is symptomatic of deeper problems, and in that case don't answer your phone outside of standby/duty periods.