If your goal is to pick up the phone connected to your OBi and easily select, which number to call out on there isn't an efficient way to do that right now. It is possible to login into your account on RingTo's website, change the default outbound call number, then pick up the phone to make a the call from that number. Obviously, doing that every time you want to select a particular outbound number is hardly an efficient process. For what it is worth, it's very easy to dynamically select your outbound number using RingTo's mobile app but that doesn't really help the OBi situation.
To do what I think you want to do RingTo would need to support sub-accounts or extensions as other providers (e.g. Callcentric,
VoIP.ms) do or the OBiTalk portal would need to allow provisioning of more than one RingTo account on a single OBi (I believe it's possible to do so with Google Voice). Unfortunately, neither of these are options at this time with RingTo.
If I'm wrong about what you are trying to accomplish, more detail might help.