Not anymore. Google has decided to relax the requirement that all calls must come in via the Google number, and is making available a range of features under its voicemail service to calls made directly to users' cell phones.

Starting Tuesday, existing Voice users will be able to link their cell phones to the Voice voicemail feature. New members will get the option to sign up for a Google phone number and all Voice features, or sign up only for the cell phone voicemail service.

The link will work via the cell phone's conditional call forwarding service, which diverts calls to a specified number if they go unanswered for a certain number or rings or if the cell phone is out of the service area. Google won't charge for the service although cellular carriers may charge depending on the service plan.

"It's a low friction way for new users to adopt Google Voice without having to get a new number that their friends family have to start dialing," said Craig Walker [cq], Google's group product manager for real time communications.

By doing this, Google liberates a portion of Voice's functionality from the restriction of the Google number and makes the service attractive to people who are mostly looking for a better voicemail service for their cell phones.