If it is not filed within the one year statute of limitations no charges will be filed. Until then, you have to call periodically to see if the case has been filed and when it is set for arraignment, at which you or your attorney will need to appear..
The government has a year to file the petty theft case. The DA may not have filed so far for a number of reasons: the DA's office has a backlog of filings and your petty theft case is not high priority for a filing deputy, the DA is still investigating your case, or the DA has already decided not to file your case. Hiring an attorney before the DA has filed may help. An attorney can convince the filing DA not to file your case. Once the case has already been filed, it will be more difficult to get the case dismissed. Good luck.
You have not stated to whom the number you were given belongs to. You want to check with the court clerk regarding status. You don't want to contact the law enforcement agency or the prosecuting agency because calling them repeatedly may "remind" them to look at your matter. You hope the one-year statute of limitations runs.