It's not impolite to say you didn't understand. In fact, by not saying it, you're implying that you DID understand. If you had asked them to repeat themselves t

It's not impolite to say you didn't understand. In fact, by not saying it, you're implying that you DID understand. If you had asked them to repeat themselves twenty times before the end of that call, they probably wouldn't have been so quick to assume that you're the problem. You might have to straight up tell them "I'm having some trouble understanding you, would you like to continue this over email?" I know your instincts are telling you that this is rude, but it's not. If they're unable to communicate via phone and unwilling to communicate via email, you've lost nothing by doing this. It would mean you're physically incapable of learning what they need from you, so you never really had a choice.