KB2UNK:

VOIPo is not particularly BYOD friendly. To my knowledge, they will provide the credentials to use your primary line on a BYOD device, such as the Grandstream telephones you are considering, but they will not provide the credentials to use the addon line on a BYOD device. So, in order to do what you want to do, you'd have to change the addon number to an entirely separate account.

If I'm wrong about any of this, someone will come along and correct me.

Rob
WA0WKO