I wanted to extend the time it took for the voicemail to kick in, so i called T-Mobile Customer Support and they gave me the following:

**61*18056377243**30#

Note: 18056377243 is the number of the voicemail box. Can be different from this.
30 is the number of seconds till it kicks in. Rep told me 30 seconds was the maximum allowed. You can put however many seconds you'd like.

Type that in the phone and then press dial. It will display a list of text, press dismiss and you should be done.

I am pretty sure that it will work on different providers.
I have seen other ways such as: *61*18005551212*11*30#
but that did not work for me. It gave me an error.

Hope this will help any of you guys.