To be clear:
Google does not condone nor recommend this procedure, but they don't police people who do it. Technically, it makes no difference which mobile phone service provider you pick. From a procedural standpoint, that mobile phone service provider needs to be willing to port in land line numbers. A few, such as Cricket, won't do it. Others, like H20, are incompetent at doing it.
The most important detail is that you will need to obtain the correct account number and PIN from the mobile carrier, which you will submit on the Google Voice porting form. Some service providers make it as difficult as possible to find this information (after all, they don't want people doing what you'll be doing - churning a port in and immediately out). If you can't find it on their website, you will need to contact that carrier. Ask them this exact question: "I need to port my <insert carrier name here> phone number out to a land line carrier. What are the account number and PIN I need to submit, so that <insert carrier name here> will approve my land line carrier's port request?" Don't get into the details that it's a port to Google Voice. Don't expect them to give you this the day after your port in is completed. Wait at least a full week.
