I am a federal employee in a hospital. My suporvisor called on Friday and asked if I could work on Saturday for a sick employee, I was told all other employees had plans, can he force me to cancel my plans to come to work? I was under the impression that the supervisior had to cover if no else was available. We have set days to be on call, but do not receive stand by pay.
Maybe I should just stop answering my phone on my days off.

Are you concerned about being disciplined or fired because you could not come in? Whether a supervisor is "supposed" to fill in or not is a matter of policy, not law.

Yes, can i be disciplined for telling him i had plans, and can he leagallly make me come to work when i am off on the weekend.

Unless you are covered by a collective bargaining agreement that does not allow this, yes, he can tell you when to work and fire you if you refuse.

thanks for your help.