partner-contact/partner_fax/views/res_partner.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record id="view_partner_simple_form_fax" model="ir.ui.view">
        <field name="name">Add fax on partner</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_simple_form" />
        <field name="arch" type="xml">
            <field name="function" position="after">
                <field name="fax" placeholder="Fax..." />
            </field>
        </field>
    </record>
    <record id="view_partner_form_fax" model="ir.ui.view">
        <field name="name">Add fax on partner</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <field name="function" position="after">
                <field name="fax" placeholder="Fax..." widget="phone" />
            </field>
            <xpath
                expr="//field[@name='child_ids']/form//field[@name='mobile']"
                position="after"
            >
                <field name="fax" placeholder="Fax..." widget="phone" />
            </xpath>
        </field>
    </record>
</odoo>