You can check which OBi is trying to make the call. If you want additional security, the sending OBi can automatically prefix the dialed number with a PIN and the receiving OBi check and remove it.
OBiTalk InboundCallRoute:
{510111111|610222222>(<PIN:>(Msp1)):sp1}
Only OBi numbers 510111111 and 610222222 can make a call with numbers with "PIN" prefix.
The default port number is 10000
There is an option TryMultiplePorts.
I don't know how this works.
Edit:
The above assumes you want to make OBi to OBi calls.
The default config will only allow others to ring the phone port, not make calls.