I always give an estimate over the phone.
I have package pricing based on outside mat size, so it's easy to give a base estimate.
I have gotten a fair amount of business because "the other guy wouldn't even give me an estimate!"
Said with incredulity!
Personally I think this is one of the biggest benefits of having a package pricing option in place. I prefer to believe that people in general are smarter than we give them credit for, and the majority of people calling for a phone price know full well that there are so many variables that it is impossible to give a realistic price without even seeing the artwork. So they're not really calling to get a firm price, rather they want to find out if they should even bother coming in. You should be able to tell a customer over the phone "Well, we have a very wide selections of frames and matting, so the price will vary somewhat. However we do have some specially-priced packages. For your 8x10 we have some options starting at $49.99."
This way your (potential) customer can walk in knowing that there's
something in your store at a very affordable price. From there it's your job to show great designs that will make them salivate and forget the budget option. But hey, even if they go for the $49.99 option you've still made a sale and won a new customer.
Also, after a while you get a feel for what things cost. You should be able to tell the caller that "We can do a typical diploma for about $X.XX dollars, or something a bit more fancy for about $X.XX." Always follow that up with "If you'd like to come by today we're open until 7:00 and we'd be happy to show you some of our options."
A few years ago Jared Davis did a talk in Las Vegas about this sort of thing, and he had some excellent tips:
Act like the customer is in your store and engage her in conversation. If she mentions that she has a diploma to frame, ask whose it is. Congratulate her, ask her where she went to school, etc. The same sort of things you'd do with a person in your store in front of you.
Tell the caller your name, and ask for hers. Tell her what your hours are and when you will be in.
Finally a tip from Vivian Kistler: Smile when you're talking on the phone. Believe it or not, it will come through.