You have an OBi, which is a VoIP Analog Telephone Adapter (ATA). You can configure it to act as the "user agent", or SIP destination, for your account with your SIP VoIP service provider. You also have a Gigaset IP phone system, which can do the same thing. You can't have them both using the same credentials (SIP user account and password) at the same time. In theory, you can configure the OBi 202 as a "PBX" to which the Gigaset can then connect, but it's complicated. I have a OBi 110 and the same Gigaset system as you. The OBi 110 doesn't support the PBX configuration needed by the Gigaset, so I gave up. The issue, if I recall correctly, is that the Gigaset can't connect to a PBX without authenticating (with a user ID and password). You can find some discussion on this forum about using IP phones (other than Gigaset) with OBi 202 boxes, if you want to experiment.
If your main concern is blocking unwanted callers, a better approach is to use the Gigaset with a better-quality SIP VoIP service provider that has blocking features you can configure on their web portal. I use Callcentric for this purpose, for example.