partner-contact/base_contact_phone_extension/res_partner_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>

    <record id="view_position_info_number_extension_form" model="ir.ui.view">
      <field name="name">view.position.info.number.extension.form</field>
      <field name="model">res.partner</field>
      <field name="inherit_id" ref="base_contact.view_partner_form_inherit"/>
      <field name="arch" type="xml">

        <xpath expr="//field[@name='other_contact_ids']/form//field[@name='phone']"
                 position="after">
          <field name="extension"/>
        </xpath>

      </field>
    </record>

  </data>
</openerp>