It is also very expensive. You are paying all that money for the fact that, as long as you are polite, people will listen to what you have to say. So make sure they get the right message. This means, above all, having a professional script, delivered professionally. Relying on amateurs is not likely to work.
Telephone scripts allow for interruptions and reactions.
Scripts should be divided in two. One side is the script as planned. The other is a series of pre-planned answers to questions and objections. The script will evolve as it is used, being adapted according to the reactions it receives. My advice on telephone is to go to professionals.