The terms ‘On-Net’ and ‘Off-Net’ are used to describe call or messaging types.
‘On-Net’ is referred to when calls or messages are made from your telephone providers network to another phone number which resides on the same telephone network. It does not matter if the phone line is in roaming or setup in a different country, calls originate and terminate on the same telephone network. This is what we call On-Net.
‘Off-Net’ applies when the call or message is made on a different network, e.g. whilst you are roaming, or if you are using your phone network and make a call or send a message to a phone number that resides with a different network provider.