The design of the SoundStation Duo is usually to utilize SIP and if the server is down it fails back to PSTN.
There is no easy way to my knowledge to toggle between SIP and PSTN but you can navigate the Menu via Menu > 3 for Settings > 2 for Advance > Enter the password > Select 1 for Admin Settings > 1 for Operational mode
If this is too many steps have a look at the EFK section:
Nov 25, 2011 Question: How can I change the functionality of a softkey button / use EFK?
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
---------------- The title Polycom Employee & Community Manager is a community setting and does not reflect my role. I am just a simple volunteer in the community like everybody else. My official "day" Job is 3rd Level support at Poly but I am unable to provide official support via the community.
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