I agree with those who have said that occassionally filling in for reception so they can get lunch or have a break and your clients can still be served is no big deal and shouldn't have you constantly in tears or feeling you are doing work beneath you, but it sounds as if there are some other issues.
I guess if you need this job and feel it would be difficult to get another comparable one, then you need to suck it up and be a team player.
Get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me. -Muhammad Ali