When using a virtual numbers as the CLI/Caller ID for an outbound call, the recipient would expect to be able to call the number back. This is common if the recipient misses the call, or is not expecting the call. It is expected that all virtual numbers used for outbound calls have a forwarding destination configured on the number.
You can find details on how you can handle inbound calls in the below article. This includes forwarding to another number, forwarding to a Voice API application, to a SIP address or a VoiceXML.
If you do not wish to receive calls from users, it is recommended to forward the number to a Voice API application with a short message advising the caller that the number is not accepting calls and where they can receive support from. An example of an acceptable message is below:
"You have been called by Nexmo. This number is not accepting calls. You can receive assistance by emailing support at nexmo dot com."
Inbound calls also use our per second billing, so you only pay for the time of the active call instead of rounding up the next minute. You can find the current pricing for inbound calls at nexmo.com/pricing.