We, Polycom, do not know the number as this could be anything. Please contact whoever installed your system as only they will be able to tell you.
Once you know it you can follow this post => here <= and then simply use the:
Or via the Web Interface use the Label parameter you can write anything you want. This can either be a DDI or the whole number.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels. Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's