neilio,
From a practical point of view, simultaneous outgoing calls can be made from: a phone attached to the OBi, plus OBiAPP softphones installed on computers.
If your question is to know how many (different) people can be using the OBi at the same time, it would be one telephone to POTS line + the softphones on the other 3 voip services (SP1/SP2/OBiTALK/LINE Port).
RonR,
I'm not certain how the Voice gateways come into play here. RonR, are those extra trunks or is 4 the limit?