Hi Pascal de Clarens,
Top left header part of default odoo report will contain contact detail of company by using t-field-options='{"widget": "contact", "fields": ["address", "name"], "no_marker": true}'
If you want to add phone number instead of country then first you have to replace the default report header.You can try below code:
<template id="external_layout_header" inherit_id="report.external_layout_header">
<xpath expr="//div[@name='company_address']" position="replace">
<div class="col-xs-6" name="company_address">
<t t-if="company.partner_id">
<span t-field="company.partner_id"/><br/>
</t>
<t t-if="company.partner_id.street">
<span t-field="company.street"/><br/>
</t>
<t t-if="company.partner_id.street2">
<span t-field="company.street2"/><br/>
</t>
//same way for city,state and zip
<t t-if="company.partner_id.phone">
<span t-field="company.partner_id.phone"/> //add phonenumber of company
</t>
</div>
</xpath>
</template>
Hope this may help you!
Regards,