Number of seconds before forwarding to voicemail

The current default number of seconds before forwarding to voice mail is 30 seconds.  I managed to change this to 20 seconds with **61*+447782333123**20# and confirmed that it had (eventually) changed with *#61#.  The problem is that it gets reset to 30 seconds every so often.

Is there a way of making the change permanent - 30 seconds is too long!