Cal. PEN § 1328d

(a) Notwithstanding Section 1328, except as specified in subdivision (c) of Section 1328, a subpoena may be delivered by mail, messenger, electronic mail, or facsimile transmission. Service shall be effected when the witness acknowledges receipt of the subpoena to the sender, by telephone, by mail, over the internet by email or by completion of the sender’s online form, or in person, and identifies themselves by reference to their date of birth and driver’s license number or Department of Motor Vehicles identification card number. The sender shall make a written notation of the identifying information obtained during any acknowledgment by telephone or in person. The sender shall retain a copy of any acknowledgment received over the internet until the court date for which the subpoena was issued or until any further date as specified by the court. A subpoena issued and acknowledged pursuant to this section shall have the same force and effect as a subpoena personally served. Failure to comply with a subpoena issued and acknowledged pursuant to this section may be punished as a contempt and the subpoena may so state; provided, that a warrant of arrest or a body attachment may not be issued based upon a failure to appear after being subpoenaed pursuant to this section.

(b) A party requesting a continuance, based upon the failure of a witness to appear in court at the time and place required for their appearance or testimony pursuant to a subpoena, shall prove to the court that the party has complied with this section. That continuance shall only be granted for a period of time that would allow personal service of the subpoena and in no event longer than that allowed by law, including the requirements of Sections 861 and 1382.