California Code, Public Utilities Code - PUC § 7938

The commission shall require, as an interim measure until the commission develops procedures for number pooling or adopts utilization standards, that number assignments made by telephone corporations to their customers shall be made first from prefixes that are more than 25 percent in use. A telephone corporation may assign numbers from prefixes with less than 25 percent use only to the extent necessary, if numbers from prefixes that are more than 25 percent in use are not otherwise available.