How to use 2 extensions with a single ISDN line


I have an asterisk installation with a ISDN BRI card.
I setup a ring group with 2 extensions for the inbound calls.
When an extension receives a call, I want the other one not to be able to receive more calls.
I want this because I have one person who uses the 2 extensions (2 phones in 2 different rooms).
I tried all the ring strategies, but with no luck.

Could you please help me?

PS: The ringall-prim is the closer to what I want, but again this happens only for the first extension in the ring group.