I am unable to call the toll-free number of my customers through Zendesk Talk. Sometimes Talk just rings and then disconnects. Other times, I reach the customer's IVR, and then when I select the department then the call ends.
You are using a toll-free number on Talk to call to another toll-free number.
Toll-free numbers are not able to call other toll-free numbers. To resolve this, use a local number on your account to make outbound calls to toll-free numbers.
To understand why this is the case, see the article: Can I forward calls from my current toll-free number to my Zendesk Talk toll-free number?
For more information about toll-free numbers, see the article: Can I make outbound calls with my toll-free number?