If you can understand her in person, can you possibly 'train' yourself to understand her over the phone? Of course, you'd have to be upfront with her about why you're doing this, and like a PP said, she'd just have to get over it if she wants to be able to call you. It's not a slight; it's a fact and a problem. It's like bad breath....it's embarrassing and a touchy subject, but if you have to be around someone and his breath makes you want to hurl, then something needs to be said, uncomfortable or not.
But anyway, about the 'training'; I used to work for a company that sold systems to hospitals. One of the doctors was not a native English speaker, and I just could NOT understand him. At all. Seriously, he sounded like the male verson of Charo, if you remember her. Luckily I was with another guy who could understand the doc. I met him another time, and then one time I had to meet with him myself. Strangely enough, he didn't speak any better, but I was able to understand everything that he said! My brain had somehow acclimated to his speech. So it can be done.
So could you have some prearranged calls over the phone with you and DH on two extensions, and just have MIL talk and talk? DH can translate, and you (and your brain) can get accustomed to hearing her. You'll train yourself. Do that a few times and I bet your understanding will improve.