necrossis,
All of your devices (the SPA, the Gizmo and the Dock-N-Talk) are FXS devices. You cannot mate two FXS ports with each other (both supplying Dial Tone and Ring Signal and neither listening), so you need to introduce either an FXS-to-VoIP device (
SPA-3000) or FXS-to-FXS adapter (SIPCPE FX-200 or FX-300) to create a path from your Dock-N-Talk to your VoIP service.