Without a VoIP service provider configured on the OBi the only* calls it can make are to other OBi devices. No calls to or from 'real' phone numbers.
You can call the echo test number **9 222 222 222 and have a convetsation with yourself.
Google Voice is one provider but there are dozens of others.
(* not strictly true as direct SIP calls can be made but that is seldom useful and way too advanced for a beginner.)