National Telephone & Communications Inc. has agreed to pay more than 10,000 customers in California up to $310,000 to settle charges that it changed consumers' long-distance phone service without their permission--a practice known as slamming. The California Public Utilities Commission had been investigating complaints of slamming and forged signatures on phone service switching paperwork. Irvine-based National Telephone agreed to pay $20 each to about 10,000 customers who said they were slammed and up to $1,000 each to about 110 customers whose signatures were allegedly forged. Regulators also put the company on regulatory probation and could take further action if complaints persist. Officials at National Telephone could not be reached for comment.