Number of seconds before forwarding to voicemail followup

[Ashcoding]

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!

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This is a follow-up to my post in Off-topic (see above).  It seems that I can set it to 20 seconds and confirm that it's been set with *#61# (status) which reports it's been changed but sets it back to 30 seconds when I do another status check?

Could someone please explain this behaviour?

[Rebekah_150]

Here are all of the codes we would advise regarding call forwarding https://help.smarty.co.uk/en/articles/1463317-setting-up-call-forwarding 

[Ashcoding]

Thanks for the link.  I tried: -

    *61*+447782333123*20#

as per the link you sent me but this gives me "Invalid MMI code".  I think there is an asterisk missing.  The correct string is: -

    *61*+447782333123**20#

which gives me "Registration was successful".

However, as per my previous post, this gets set back to 30 seconds by Smarty/Vodafone.

[Ashcoding]

Sorry - should read Smarty/Three.