Congrats on the interview. Case interviews over the phone are pretty common. A couple of scenarios I can think of:
1. A very open-ended question with minimal interviewer interaction. For example, "how many phone calls are made each day in the US?" This could be something that the interviewer just comes up with on the spot, so I would prepare by just thinking of very random things you could estimate and then working through the thought process. It also helps to know some very common data such as the population of the US.
2. A 'guided' case with basic math. This could be something like, "Client XYZ works in 15 regions and is trying to optimize his sales force for these regions. He has 10 sales associates in each region making an average salary of $100,000 per year." These are usually fairly straight forward, as the interviewer is just looking that you understand basic business principles and can think under pressure. The only math you might have to do would be something like "I recommend reducing the sales force in region Z by 2 associates for a savings of $200,000." It will probably be a little more in-depth than that, but hopefully you get the picture.
Overall, I wouldn't stress too much about the case if you already feel comfortable with process thinking and working under pressure. Hope that helps.