Look at it this way—would you rather spare your employee's feelings, or send potential clients to that law firm down the block? Proper telephone demeanor is a key component in promoting your law firm.
If your paralegal winds up losing her job because your firm goes out of business, that won't do much for her feelings, either, and you may be joining her on the unemployment line.
Part of the baggage that goes with being the managing partner of a law firm is having to nip inappropriate behavior in the bud, which usually entails having some mildly uncomfortable conversations.
Yes, you need to inform your paralegal that she has to present a more professional image on the phone, and you have no choice but to sit down with her, describe what you've seen on the surveillance tapes, and suggest ways she can improve the client experience.
You may be tempted to skip the face-to-face meeting and simply send your staffer an email, but this is a very bad idea!
If you present your critique the right way—not as a head-on assault, but as a series of gentle suggestions—your paralegal will be more likely to absorb what you have to say and change her behavior going forward.
For all you know, she already senses that she isn't handling phone calls as well as she could be, and will welcome this opportunity to improve her performance—and if she doesn't, you'll know that you don't have a “team player” and that this person needs to be reassigned to a place far, far away from the phone banks.