connector-telephony/base_phone/web_phone.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  © 2014-2016 Akretion (Alexis de Lattre <alexis.delattre@akretion.com>)
  License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
-->

<odoo>

    <template id="assets_backend" name="base_phone assets"
        inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript"
                src="/base_phone/static/src/js/phone_widget.js"></script>
        </xpath>
    </template>

</odoo>