Thanks guys for the suggestion. It turned out that connecting the OBi100 directly to the modem was nearly impossible. The reason is that every device that connects to the modem needs to login to my ISP and the ATA can't do that. I tried several workarounds but nothing worked.
I decided to look at the configuration of my SPA3000 which luckily still works. After porting the settings to the OBi100 I can finally make and receive calls. Here are the settings in case others have the same problem.
[Generic Service Provider]
Service Provider Proxy Server:
sip.broadvoice.comService Provider Proxy Server Port: 5060 (default)
Outbound Proxy Server:
proxy.xxx.broadvoice.com (where xxx is one of lax, dca, mia, atl, chi, bos, nyc)
Outbound Proxy Server Port: 5060 (default)
User Name: 10 digit Broadvoice number (e.g. 8009990000)
Password: 10 character Broadvoice password (from tech support, e.g. Gh2Fr1am2c)
URI: *blank* (default)
I'd like for OBi to update the website to save users the hassle of figuring out what the settings should be. Especially since Broadvoice is a supported service.