website_mass_mailing_sms/data/ir_model_data.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data>
        <function model="ir.model.fields" name="formbuilder_whitelist">
            <value>mailing.contact</value>
            <value eval="[
                'mobile',
            ]"/>
        </function>
    </data>
</odoo>