Keep this somewhere you can find it. The point of writing it down is that you never have to wonder whether your phone is broken.
Stop the divert
Dial ##002#. That cancels every diversion on the handset.
If you would rather cancel one condition at a time: ##61# for no-answer, ##67# for busy, ##62# for unreachable.
Your own number is untouched by any of this, and always was. There is nothing to port back, no settings on your account to reverse, and no notice period on the number itself.
Put your voicemail back
Telstra: **004*101# points every condition back to MessageBank. Message2Txt starts working again once MessageBank is back on.
Optus and Vodafone: re-enable voicemail in the carrier app.
If something looks wrong but you have not cancelled
Ring your own number from a different phone and let it ring out. Then dial *#61# to read back what is actually set. Most problems are one of two things: a divert that was never set, or ##002# run after the divert codes instead of before, which silently cancels them.
If the codes read back correctly and it still is not catching, tell me and I will look at the call records.