The correct way to format a "61" command is this.

You will need to know your voicemail phone number (UK T-Mobile is 447959118207) the 44 is the country code and you drop the first zero of the standard voicemail number if applicable.

So the correct command will be:

*61*447959118207**25#

Then press send.

The 25 at the end, is the number of seconds before diversion to voicemail. up to 25 seconds.