Of course the programme needs to be covered, but that's managements problem, not yours. If needs be, they should recruit more pilots, not screw around the ones they have.
The solution to calls on day off is very simple, buy a pay-as-you-go phone, give that number, and that number only, to work and only switch-on on standby days. Problem solved.
Persumably bmi will reimburse all these calls from personal mobiles...