odoo16/addons/website_sms/views/website_visitor_views.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo><data>
     <record id="website_visitor_view_form" model="ir.ui.view">
        <field name="name">website.visitor.view.form.inherit.website.mass.mailing.sms</field>
        <field name="model">website.visitor</field>
        <field name="inherit_id" ref="website.website_visitor_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//header" position="inside">
                <button name="action_send_sms" type="object" class="btn btn-primary"
                        attrs="{'invisible': [('mobile', '=', False)]}" string="Send SMS"/>
            </xpath>
        </field>
    </record>

    <record id="website_visitor_view_kanban" model="ir.ui.view">
        <field name="name">website.visitor.view.kanban.inherit.website.sms</field>
        <field name="model">website.visitor</field>
        <field name="inherit_id" ref="website.website_visitor_view_kanban"/>
        <field name="arch" type="xml">
            <field name="page_ids" position="after">
                <field name="mobile" widget="phone"/>
            </field>
            <xpath expr="//div[hasclass('w_visitor_kanban_actions')]" position="inside">
                <button name="action_send_sms" type="object" class="btn btn-secondary"
                        attrs="{'invisible': [('mobile', '=', False)]}">SMS
                </button>
            </xpath>
            <xpath expr="//div[hasclass('w_visitor_kanban_actions_ungrouped')]" position="inside">
                <button name="action_send_sms" type="object" class="btn btn-secondary border"
                        attrs="{'invisible': [('mobile', '=', False)]}">SMS
                </button>
            </xpath>
        </field>
    </record>

     <record id="website_visitor_view_tree" model="ir.ui.view">
        <field name="name">website.visitor.view.tree.inherit.website.sms</field>
        <field name="model">website.visitor</field>
        <field name="inherit_id" ref="website.website_visitor_view_tree"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='is_connected']" position="before">
                <field name="mobile" invisible="1"/>
            </xpath>
            <xpath expr="//button[@name='action_send_mail']" position="after">
                <button name="action_send_sms" type="object" icon="fa-mobile"
                        attrs="{'invisible': [('mobile', '=', False)]}" string="SMS"/>
            </xpath>
        </field>
    </record>
</data></odoo>