Videx 3131 feedback noise

Have wired up my Ring intercom to my Videx 3131 intercom as per the instructions. When someone rings the bell I get the notification on my mobile and am able to answer, however I get a lot of feedback noise both on the mobile and on the outside unit. Opening the door works fine too. When answering from the intercom handset it is clear. What could be the issue? Have also tried lowering microphone and speaker from app settings but to no avail.