I had the problem of Google Voice in SP1 showing for 3 or 4 days continuously and SP2, SP3, SP4 not showing "Registered". But I was able to use the SP2 for calling, OBI to OBI calls could not be made.
I tried by rebooting the OBI Device and Router, but still google voice was showing as "Offine". But then I noticed that the Lan cable was not fully plugged in the router. When I pushed it fully inside Google Voice in SP1 changed to "Connected"and all other SP2, SP3, SP4 changed to Registered. After that I could make calls to other OBI devices and all other SP ports could be used to make calls.
If anybody have the same problem check first that all connections are properly done before trying to hard reset the OBI device or the router.