Federal regulators have spent much of the past year prodding Internet phone providers to link their customers to the enhanced 911 system, but accomplishing that goal is no small feat, industry representatives said Tuesday. The enhanced 911, or E911, system is a step up from the basic 911 system in that it supplies emergency call operators with the caller’s geographical location and callback phone number. The Federal Communications Commission has decreed that such E911 service must be mandatory and new customers will be denied the option of choosing VoIP (voice over Internet Protocol) service without it.