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The central register for area code + prefix is here:

http://www.nanpa.com/reports/reports_cocodes_assign.html

However, sometimes the companies listed are subsidiaries and you can't tell the 'major' cellular provider for someone at a particular number.

In the US, does number portability between networks exist? In NZ, it causes problems with SMS gateways, which seem to (in general) naively use the prefix as the sole means of identifying which operator for a number.

Most VoIP providers have a "number lookup" feature. For example, here is my number looked up on CallWithUs:

http://cl.ly/image/1l1g071h1d0C

You can create an unfunded account with them and use the lookup feature, if I'm not mistaken. (Though I don't recommend doing so, it's not nice!)

Most of US-based do. No such luck in here (Czech).