This happens because your OBi number is in their Circle of Trust.
Any OBi number can call any other OBi number. If the Obi that you call has your OBi number in their Circle of Trust, then you get their Auto Attendant which gives you options that allow you to use their outgoing services.
If you have no need of onward dialling etc using your friends' OBi devices, then simply get them to remove your OBi number from their OBi Circle of Trust. This can be done from their OBi Dashboard page.
If you and your friends wish to stop the Auto Attendant from answering, but keep single-stage through calling using each others' OBi devices, then all rules containing "aa" should be removed from:
Voice Services > OBiTALK Service > InboundCallRoute
