I really feel that you can try a technique to improving selling skills. Let them do an exersise of selling (one to one) . That way they would develope a confidence to do telemarketing which is very much easier.
Thats my personal experience.
Also all of this would come in a package for a week long “Selling Workshop” by some eminent trainers. I really feel thats a skill to be born with . Developing it requires lot of motivation and dedication from the learner which can come if the workshop is effective. And the workshops should be done pretty often to retain the learning.
Hope that helps you….