It is clearly unsupportable nonsense to suggest that "everyone' has a mobile phone. Until that is a requirement of passing your medical that cannot be the case.
Even if crew do posess one the company cannot dicate that they carry it when not on duty.
Equally they cannot require them to remain in areas of mobile coverage when not on duty.
If they choose to so constrain the crew outside a duty period as above then the employee is clearly acting at the employer's behest and is thus on duty.