Google yesterday announced that Google Voice customers can now port their mobile number to the service, but there are a few catches.

According to a blog post on the company’s website, Google Voice customers can now port their carrier phone number to the service for a $20 fee, in addition to any early termination charges incurred on the carrier side of things.

Until now, Google Voice users have had to use a unique phone number assigned by Google. Porting a number to Google Voice allows users to use the service without having to alert their contacts of the new number.

While it may sound cut and dry, the devil is in the details. There is that one-time $20 fee to port your number, and Google tells users that they may experience disruptions in sending and receiving text messages for up to three business days after the porting process is complete.

Remember, “Google is not a mobile phone service provider,” as the blog states, and in order to complete the porting process, users have to go back to their carrier and set up a new mobile phone service plan and request a new number. Once the user has secured a new mobile service plan and a new number, they need to add that new number to their Google Voice account as a forwarding phone.

If a user already has a Google number on their Google Voice account, it will be replaced by the number that they have ported and will be completely removed from their account 90 days after porting is complete.

Google says that number porting is currently available for existing Google Voice users and will become available to new users within the next few weeks. Google Voice is available in the U.S. only.