The general answers to your two questions are no and no.
Regarding the first, how would the OBi know that you have used your 300 minutes of allocation?
As to the second, depending on what you mean by "goes down" you might have some success by putting SP1 and SP2 in a Trunk Group and sending outgoing calls to that.
http://www.obihai.com/OBiDeviceAdminGuide#_Toc367543101