partner-contact/partner_phone_extension/views/res_partner_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <!-- Add phone number extension fields for partner -->
    <record id="view_partner_simple_form" model="ir.ui.view">
        <field name="name">view.partner.simple.number.extension.form</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_simple_form" />
        <field name="arch" type="xml">
            <field name="phone" position="after">
                <field name="phone_extension" string="Extn." />
            </field>
        </field>
    </record>
    <record id="view_partner_form" model="ir.ui.view">
        <field name="name">view.partner.number.extension.form</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <field name="phone" position="after">
                <field name="phone_extension" string="Extn." />
            </field>
            <xpath
                expr="/form/sheet/notebook/page/field/kanban/field[@name='phone']"
                position="after"
            >
                <field name="phone_extension" />
            </xpath>
            <xpath
                expr="//div[hasclass('oe_kanban_details')]/div[@t-if='record.phone.raw_value']"
                position="after"
            >
                <div t-if="record.phone_extension.raw_value">
                    Ext:
                    <field name="phone_extension" />
                </div>
            </xpath>
            <xpath
                expr="/form/sheet/notebook/page/field/form/sheet/group/group/field[@name='phone']"
                position="after"
            >
                <field name="phone_extension" string="Extn." />
            </xpath>
        </field>
    </record>
</odoo>