Please help, when switching from the Sigma topic to Starter, the phone numbers of old ads disappeared, when viewing from the admin, the numbers are there, if you create new ones, then the phone number is visible. How to fix this problem. Thank you!

Sigma theme use Osclass item table, while Starter will use t_item_starter table to store phone numbers.
You can copy it over or update function that get's phone & save it.