Hey @smcs
This PR should add what you want to the API. You can lookup contacts via mobile or phone by specifying the "contact_phone_or_mobile" parameter. This should be handy for other systems too, hopefully.
I tried to test it with 3CX using their CRM Integration Tool but all it seems to do is generate an XML rather than actually test.. I believe the URLs you want for 3CX are:
http://itflow.example.com/api/contacts/read.php?contact_phone_or_mobile=[Number]
http://itflow.example.com/api/contacts/read.php?contact_email=[Email]
http://itflow.example.com/api/contacts/read.php?contact_id=[ContactID]
Here's what their tool generated for me anyway, in case it helps: https://pastebin.com/FPwGneqH
I'd recommend starting using PowerShell (see here) or Postman to verify your lookup is working properly, and then moving onto their tool/integration.