Hi Manjo,
Thank you for reaching out to us in our community, I am sorry you haven't been able to extent the rings on your handset.
To extend the ring time to 30 seconds, first enter this code into your phone: **61*+447458333222*11*30#. Next, press the call or answer key - your phone will do the rest. You can alter this to ring for 5, 10, 15, 20 or 25 seconds by replacing **30# with your choice of seconds.
Kind regards
Paul.