You mentioned the main point is not on improving their overall English. However, I think that is precisely the point. You should either hire people
with better Engish skills or train and teach English to your employees.
I recommend an intensive English learning program, based on telephone
talks. You should hire some Americans (just one or two) to talk to them
by phone, about 20 mins per day.
Regarding learning material, you should make it yourself. For example: you could record some real telephone calls and transcribe the text. With the text transcriptions you may write lessons, focusing on grammar points, listening comprehension, interpretation and so on. Do it yourself or hire someone to do it for you.