Hi @belasor ,
We are unable to use “tel:” links in Text editor in SharePoint Online currently.
As a workaround, you can configure a Modern Script Editor web part and use <a> to achieve your requirement. The below lists the steps for your reference:
1. Please refer to this article to configure a modern script editor web part:
https://social.technet.microsoft.com/Forums/office/en-US/d4b8b3c4-8950-4407-9120-204b80fc35f9/sharepoint-online-best-practice-install-modern-script-editor-web-part-for-sharepoint-online?forum=odspproductsandservicesharing
2. add the HTML code into the Modern Script Editor web part:
<a href=”tel:XXXXX″>Phone Number</a>
Also, there is a user voice about this similar topic, you can vote up it.
https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/41062174-enable-sip-and-tel-links-in-editor
I hope this information has been useful, please let me know if you still need assistance.
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