I'm not sure of a way when using full integration, which features do you use in Google voice?
i like the online features for seeing texts online (free), ability to do "do not disturb" (free), voicemail transcripts (free), the fowarding options, and the various call options for different contacts.
I thought it may have something to do with the Conditional call fowarding settings?
Since you have integrated your sprint number into the Google voice account many of the network settings are based on a preset configuration, which phone do you use?
I cannot find a way to use Sprint's visual voicemail with Google voice, while it's integrated with your sprint phone number. the only way to use sprint's voicemail, and still use google voice, would leave you without the ability to manage unwanted callers through google voice. You should consider that not using Google voice for voicemail will limit you to only one outgoing message, limit the transcription delivery options, and keep you from seeing calls/voicemail online.