If the caller (you, in this case) hears ringing, but the GV number isn't ringing, then you've either de-selected Google Chat as a forwarding destination, or you have set some specific calling behavior for that number, such as not ringing any of your forwarding phones, as LTN1 pointed out.
A special case is when calling into your GV number from one of your defined forwarding phone numbers (a number that is on your list of forwarding phones, on this page:
https://www.google.com/voice#phonesYou can decide: when calling your GV number from that forwarding phone number, do you want your GV number to ring, so it can be answered, or do you want to get directly into your voice mail menu tree, so you can listen to your messages? This is known as the "Voicemail access" option. To change this setting, click the Edit button under each forwarding phone number, then click "Show advanced settings". The default behavior for numbers classified as "mobile" to go directly to GV VM, and for numbers classified as "work" or "home" to ring the GV number.
If you have further GV-specific questions, I recommend posting them over on the Google Voice help forum (URL is in my signature block below).