You imply that you have a problem by just attaching the OBi to your network, let alone trying to make a call using the OBi.
Does the OBi obtain an ip address from the modem you are attaching it to or from an external router that is attached to the modem? Can you access the OBi configuration by attaching a computer to the OBi202 router's LAN, or to an external router that both devices are attached to?
I would document your bandwidth available by running several bandwidth speed tests using a service like
http://www.speedtest.net/ from your computer at different points in time.
I would try to pin down what feature of the OBi is causing the problem with your "not very reliable" dsl service. If you have a better idea of what is causing the problem then maybe there is a way to mitigate the problem.
I would try to see if the problem is occuring with the OBiTalk Provisioning, the OBiTalk Service, or the configurations you have setup either on SP1, SP2, SP3, or SP4.
You can disable/enable the services and run some tests.
As you know, if you configure the OBi manually on the OBi web page you need to disable the Auto Provisioning as outlined in this thread:
http://www.obitalk.com/forum/index.php?topic=61.msg109#msg109