IF the main idea here is to force the user to make a conscious decision to provide or not provide a phone number then maybe you could integrate a radio button group into the control.
The first radio button could live alongside the text box for the phone number. Selecting this one would mean they then have to provide some content in the phone box.
The second radio button could have a label saying e.g. "I do not want to provide my phone number".
This approach could work because on load neither radio buttons would be selected and the majority of users are used to having to make a choice when presented with a group of radio buttons. Therefore upon pressing submit they may not be surprised to be bounced back to this control to make a choice.
Also you are forcing them to make that active choice not to provide the phone number, which takes a little more thought than simply leaving a field blank.
Upon selecting the "I don't want to provide it" option, you could disclose a subtle message reminding of the terrible consequences of not providing one e.g. "Are you sure you don't want us to have your number? We could add you on WhatsApp if you do". They may then get scared and change their mind and provide one.
Basically, provide a more clear approach to what is required/not required but with a little twist as to maybe get more people to provide the info you want but don't require.