Scenario: It seems many MVNOs cut out some of the more useful features on devices. i.e. VVM. My primary number doesn't matter to me, whether it be a Google allotted number or my mobile service number, so in using the Google Voice number, it seems that if a carrier isn't an officially supported network , Google's support docs are limited, subscribers are still able to forward calls and have google voice services so long as users let the call ring out. If the subscriber proactively selects "do not answer" then the service routes to carrier (in my case a GSM MVNO). It seems the MVNO's system may or may not grab the call, depending on whether or not its own ring period is longer or shorter than 25 seconds. Here is my current MVNOs "not so clarifying" help site article.
Is there a way to rely more on the forwarding feature to enable Google Voice's service when the user selects "do not answer"?