UPDATE:
So I called. On the Awkward meter, it rated about a 5. Fortunately, her husband had remembered to tell her, so she knew my name and was expecting the call. Unfortunately, she was on her cell (the only number I was given) and was out and about, so couldn't really talk - slightly awkward, but she did choose to answer the phone.
Anyway. I wasn't worried about handling it - I'm a grownup, after all - but still it think it was a weird way to go about it.
Lady Disdain, I think Bibliophile was addressing another poster that suggested I not call; I don't think she was addressing me.
I do think it's a bit odd for the invitee to call the hostess to
arrange an event.
But, then again, it was really a request for an RSVP, wasn't it?
King was given the invitation. Host gave him a number for Queen to call back and respond.
The only awkward thing is that it wasn't a set-in-stone invite. As in it wasn't these are the plans, this date/place/time, will you be there or not? Not the usual invite, but an invite nonetheless and something that needed to be RSVP'd to.
Queen -- if you are willing, I'd like to hear more details concerning the conversation with the hostess. Did you discuss a date when you could get together? Did she ask you about preferences as to restaurant, time, etc.? Did she have a plan? Was she willing to work around your schedule?
I know you said she was out and about . . . Just curious.

No, I don't think that anybody was rude. It was just an unusual situation.