If you keep your phone calls brief, your subjects are more likely to take your successive calls keeping a most important line of communication as open as possible.
Try to establish in the minds of your subjects that all telephone calls from you or to you are brief. This is accomplished by actually making sure that all calls from or to you ARE Short. Maximum 5 or 6 minutes under normal circumstances. Much shorter if possible.
This technique will establish you in the mind of your subjects as a no-nonsense business person who values his time. The kind of person the widest spectrum of subjects want to do business with.
The type of call your subjects are probably going to take not pass on.
Remember, nobody wants to take a call from anyone known to keep you online longer than you are comfortable with.