The issue here is the limited display, and configuring the handset to suit your requirements. In this case if you leave the External Mask blank, then by default the DN of line will appear at the top right of the display. The limitation here is whether you require external CLI based on the External Mask on certain DDIs or the main number? If you require different CLIs then you will need to look at Calling Party Transformation Patterns applied to your outgoing gateway transformation Css.
Please refer to following link for further information around Transformation Patterns: