partner-contact/partner_phonecall_schedule/views/res_partner_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017 Jairo Llopis <jairo.llopis@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->

<odoo>

    <record id="view_partner_form" model="ir.ui.view">
        <field name="name">Partner phonecall schedule</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form"/>
        <field name="arch" type="xml">
            <xpath expr="//notebook">
                <!-- `many2many_tags` widget would not allow the user to click
                     and edit a calendar record as the list widget does;
                     management menus are under global settings, so only
                     admins could do it; we need plenty of space to display
                     the aggregated schedule. All that said, we have to use a
                     list widget inside a dedicated page. -->
                <page string="Phone calls" name="phonecalls">
                    <group>
                        <field name="phonecall_available"/>
                        <field name="phonecall_calendar_ids"/>
                        <field name="phonecall_calendar_attendance_ids"/>
                    </group>
                </page>
            </xpath>
        </field>
    </record>

    <record id="view_res_partner_filter" model="ir.ui.view">
        <field name="name">Partner phonecall availability</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_res_partner_filter"/>
        <field name="arch" type="xml">
            <xpath expr="//separator[last()]" position="before">
                <filter
                    string="Available for phone calls now"
                    name="phonecall_available"
                    domain="[('phonecall_available', '=', True)]"/>
            </xpath>
        </field>
    </record>

</odoo>