If this woman is your manager, you can always talk to her about it. It doesn't make a lot of fiscal sense for someone at a manager's salary level to give up 2-3 hours of work time to fill in for a receptionist. Otherwise, you're going to have to suck it up. Going to HR or complaining about it will only make you look bad.

I understand how you feel. I'm in marketing and people tend to view us as glorified admins. There's nothing wrong with being an admin but that's not my job, nor is it something I'm good at.
When are women going to face the fact that they donít know their own bodies as well as men who have heard things?

Don Langrick
Bonsai Culturist