You can't register two VoIP phones with the same VoIP account, that is why you are probably encountering problems.
To share one VoIP line with multiple VoIP phones/software (such as IVM, Express talk, your SPA9000, etc.) you need the Axon software (Free, and also made by NCH).
It will register with your VoIP account and let multiple devices/software phones use it to answer and dial out calls. It does this by creating multiple extensions / your own internal VoIP accounts to setup with each VoIP adapter and VoIP software you are using. So, in the end, each VoIP device or software has its own VoIP account to register with.