Why am I prompted to "Contact customer service to activate" when placing an outbound call?

In this article:

If you've received a message requiring your to contact customer service to activate when placing an outbound call, it's likely that the message also indicates the cost for the call.

In order to complete this call, you will need to activate international calling on your account. The following articles will guide you in this process:

Step by Step Guide: Buy a new number and forward to an international number

Step by Step Guide: Port a number and forward calls to an international phone number

What are your rates to forward calls to my international number?