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-activation for AnyDevice needs to be configured on the XPhone Connect Server. 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 "Tel. Mobil" of the User's XPhone Connect Server.
3.) Licenses of XPhone Connect Office Plus need to be available for User of the Mobile App.
If all requirements are met, a blue button will appear in the Mobile App.