To take advantage of Google Voice's telephony management features, users have had to fulfill a tricky requirement: convince their contacts to call them using the single "phone number for life" that comes with this free online service.

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.

Those people will get the option of adopting a Google phone number, and thus get the full menu of Voice features, later, if they choose to, Walker said.

Some voicemail features include text transcripts of messages, the ability check messages online or from a phone, receipt of messages via e-mail or SMS and the ability to assign different greetings to different callers.

Calls made to the Google-provided phone number can be routed to all or some of a user's other lines, as well as managed in various nifty ways.

However, Voice users are finding that many contacts insist on calling them on their discreet numbers -- cell phones, office lines, home numbers -- thus bypassing the Google service and its handy-dandy call management features.

Then there are Voice users who only or mostly use their cell phone for all or nearly all their calls, and thus don't have the problem of juggling multiple phone lines anyway, Walker said.

The reason why the voicemail feature is available only for cell phones is that they have a standard call-forwarding method, while landlines do not, he said.

Google Voice, called GrandCentral when Google acquired it in 2007, is available only by invitation. Those interested can request an invite from Google, or from a friend who has a Voice account.