Here is the deal, being afraid they will never call back is not a good reason. This is FEAR OF REJECTION talking to you.
I leave voicemails. Not boring, average voicemails. When I am on the phone, I am excited. I am talking fast and having fun. I am fired up. When I leave a voicemail, I let it rip. I promise not to waste their time and tell them to call me.
Now, do many of them return my call? NOPE. So why leave a voicemail?
I just gave them a commercial! I follow up relentlessly and I will not stop calling someone until I can pitch them directly. If they are even a little interested I continue to call and send letters until I get an appointment.
You can't let something like voicemail stop you. If you leave them 30 voicemails, they will eventually call you back or hear you out!!
You just have to attack, attack, attack like a hungry dog chasing a hot dog truck.
Become a machine. Let nothing stop you. Follow up relentlessly. Persist until they either cuss you out or they hear you out. If they are hesitant to make an appointment but don't hang up or tell you no, continue to follow up with them until they agree to see you or they get a restraining order against you!
It can all begin with a voicemail, and it is just another opportunity for them to hear from you.