The Call History of your OBi will show that.
Point a browser to the IP address of the OBi on your LAN. If you don't know the address dial ***1. Log in with username admin and, if you have not changed it from the default, password admin. On left hand side click on Status, then on Call History. Look for the time of the call. You should see both the incoming call to your OBi and its outgoing call to your cellphone.
(If you have an Obi202 or Obi212 and the above does not work, come back here and say so.)