- Let's first compare (a) "If Mrs. Hillier calls," (b) "If THE Mrs. Hillier calls," and (c) "If A Mrs. Hillier calls."
Normally we don't use articles with courtesy titles. So we would expect you to say form (a). If Mrs. Hiller calls, tell her so-and-so.
But suppose that in my apartment building Mrs. Ann Hillier lives in Apartment 1A and Mrs. Betty Hillier lives in Apartment 2B. If you bring a delivery and I can discern that you should deliver it to Mrs. Ann Hillier, I might say "You want the Mrs. Hillier in 1A." Where I'm trying to distinguish one from another, I might use form (b). But that doesn't seem to be a consideration in your example.
Now suppose that I only know one Mrs. Hillier, but you don't know her. I want to tell you what you are to tell Mrs. Hillier, but I know that her name will mean nothing to you. I might say form (c), "If a Mrs. Hillier calls," meaning "If anyone named Mrs. Hillier calls." I do this to keep you from asking me "Who is Mrs. Hillier?" By using form (c) I'm telling you that I don't expect you to place the name.
- Next let's compare (d) "I'm away on trip," (e) "I'm away on the trip," and (f) "I'm away on a trip."
I cannot actually think of a situation in which form (d) would be correct. The word "trip" seems to require an article. But which article?
If Mrs. Hillier and I have been discussing a particular trip, and she will know the details without any explanation, it would make sense to say that I'm away on the trip, the trip we were discussing.
But if that's not true, and if (for example) I'm not actually on a trip at all but just asking someone to say falsely that I am on a trip, then form (f) would be most natural. Say that I'm on a trip, some trip or other.
So it seems like the best course here would be: If a Mrs. Hillier calls, say I'm away on a trip.