Hello! I noticed that the new version 8 osclass has a new Custom Field - Phone.
But at the same time, the Phone field is already built into the Sigma theme. How does it fit together? The phone from the Sigma theme is recorded in which field? Thanks

@alex7070
Please check HTML input types:
https://www.w3schools.com/html/html_form_input_types.asp

Does not mix "type" of field and "meaning" of field.

You misunderstood me. If I add an additional field Phone, then when using the Sigma theme, 2 fields for the phone are visible there. And when using other themes, for example Bender - 1 field, the one I added.

If osclass already use phone field and store phone in item table, what is purpose to create another one?
In some edge case it is needed to have ie multiple phone number, but suppose in such case you would not ask how to use it 

If osclass already use phone field and store phone in item table, what is purpose to create another one?
In some edge case it is needed to have ie multiple phone number, but suppose in such case you would not ask how to use it 

The problem is that if I want to change the Sigma theme to another, then all the phones that users entered on the site with the sigma theme will not be visible in the other theme.

Sigma theme use osclass item table.
Themes use their own.
Migration may be needed after theme change

In general, the telephone field is the most important in the ad. It is not clear why this field was not made standard in all themes.

Because it was not available on item table in past

Maybe now it is possible to oblige  authors of all themes  to use the telephone field so that there is no further confusion?

You cannot force anyone to do it and until theme has own solution, there is no real reason to do that 

Or maybe it is possible to make the functionality of osclass itself to force the display of the phone field regardless of the theme used?

Each theme usually ise own table, so ultimate solution does not exists.
Ideal way is to migrate phone data from theme tables to item table.
One simple sql should do it