If your friends live in a different country, then they can call you back via a local number (from 'By Dialing' section), and they will reach you on your phone, while it will be a local call for them and you will pay for the international rate.
It is important to note that the contact has to call from the same phone number to which you originally dialed.
To enable the feature login to your account on Voxofon.com
Open contact details page > Check "allow this contact to call me by dialing a local number, I will pay for the call" option. The local number will be generated under "By Dialing" option.