You need an attorney to subpoena someone in court. This is not something you can do yourself.
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Contact the party or attorney that issued the subpoena to you. If you are state's witness, contact the prosecutor's office. If a private attorney has called you to be a witness for their side, contact that attorney. You are being called to serve as a witness for one side or the other, and that side is responsible for all costs and fees associated with your appearance and testimony.
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If you were served with a subpoena then you need to appear.If you were served with a subpoena then you need to appear.If you were served with a subpoena then you need to appear.If you were served with a subpoena then you need to appear.
You will need to have an attorney prepare the subpoena and a process server deliver it to the cell phone company.
If it exists - have your attorney, or your insurance company's attorney, subpoena it.
In California, attorney are authorized to issue subpoenas to in-state witnesses. However, in order to obtain a subpoena of a witness who is out-of-state, the person seeking the subpoena must get an order signed by the judge.
File a motion to quash (rescind) with the court where the subpoena was issued. If the judge agrees that the subpoena has no merit he or she will declare it invalid. Usually filing such a motion is the only way to have a subpoena rescinded other than the attorney who issued it voluntarily canceling it.
The subpoena should have contained all the necessary information on when and where to report and/or any other necessary information. Read it, or contact the Clerk of the Court or the attorney which issued the subpoena.
If you have an attorney representing you, he or she can create the subpoena and have it served on the witness. If you don't have representation, you must ask the court to issue the subpoena. You may then be required to pay the costs for a process server, constable, or deputy sheriff to serve the subpoena on the witness.
(I am not a lawyer. This is not legal advice.) The answer almost certainly depends on the rules of civil procedure for the jurisdiction in question. ...but that being said, in many cases the subpoena will need to be signed by either an attorney licensed to practice before that court, or the clerk of court. If so, you can get a blank, signed form from the clerk. Then, when you serve the subpoena, you must generally also file a copy of the subpoena and a "certificate of service" (documenting how and when you served the subpoena) with the court. This is presumably to establish a timeline, in case you need to petition the court to compel enforcement of an unanswered subpoena.