Automatically determine timezone using phone number - looking for database mapping

Question

Hi,

We require phone numbers from our customers for several reasons, and we need to know what timezone they are in for calling them. I was thinking of automatically setting their timezone based on their area code.

Is there a free database I could import which maps area codes to timezones? I know there are several web sites you can query, but I would like it in our own local database table.

PS. I know that because of VoIP and cell phones, this won't always be reliable, but for those cases we will just have to manually change their timezone.

Thanks.