You can't use the One Number function via the XPhone Connect Mobile App. To use the One Number function (outbound) for the Mobile Integration of the XPhone Mobile App you need to meet the following requirements:
1.) The XCC-configuration for AnyDevice on the XPhone Connect Server needs to be completed. For reference, look at the activation instructions for "XCC" for the PBX used by the customer listed in https://help.c4b.com/en/xphone-connect-6/admin/index.htm#437.htm
2.) There needs to be a number registered in the field "Mobile phone number" in the user's XPhone Connect Server data.
3.) Licenses of XPhone Connect Office Plus need to be available for the user of the Mobile App.
If all requirements are met, a blue button will appear in the Mobile App.