Use the dialer in Hangouts (Android's default messaging app), and calls to US and Canada are free. VOIP is the answer to free or nearly free calls with all your customers. You can get an 800 number for ~$2/month to make it toll-free for them.
The $2/month, where can I find that?
All in all, I want to talk with my customers and do a tech demo (would need a tool to share screen as well).
There are also numerous great conference/screen share tools out there, like speek.com