How are you using the OBiTalk service? Although it presents some functionality difficult to obtain with SIP, I have a general distrust of free services and have designed my own system not to use any. With careful design, it may be possible to get the desired feature set without OBiTalk.
Alternatively, you might use another service as a backup. For example, if you use OBiTalk to make calls on the landline connected to a remote OBi (with single stage dialing and automatic routing), your systems could also be interconnected via a free or paid SIP provider. Calling through that path would involve going through the AA and be less convenient, but that's better than not being able to call at all.
As I write this, my OBi also sees the OBiTalk service offline.