In reply to DaveHK:
The 9-5, Monday-Friday response from MRTs generally comes from shift workers, the retired and the self-employed. Luckily most jobs happen at weekends or in the evenings anyway.
Lots of professions/jobs won't release their paid staff to bugger off up a hill to wear red and fling O2 at ramblers, it's just part of the reality of volunteering.
Attendance at training, callouts and fundraising events normally has a minimum requirement, but this varies greatly between teams.
One thing for all prospective MR volunteers to remember is that it's not just you giving time to the team, it's your family too. They're the ones you need to talk about your availability, not your employer IMO.