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_number_extension_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_short_form_extension_form" model="ir.ui.view">
        <field name="name">view.partner.short form.extension.form</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_short_form"/>
        <field name="arch" type="xml">
            <field name="phone" position="after">
                <field name="phone_extension"  string="Extn."/>
            </field>
        </field>
    </record>

    <record id="view_partner_number_extension_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>