Signing up does not switch it on by itself. There are four steps, and the last one is the one that matters.
1. You get a number
I buy you a dedicated Australian mobile number and wire it up. You never advertise this number and you never give it to a customer. It exists only as the place your missed calls go.
Your own number is not touched at any point. Not during setup, not while it is running, and not if you cancel.
2. You point your missed calls at it
You set call diversion on your existing mobile so that calls you do not answer land on the new number instead of your carrier voicemail. This is a few codes dialled on your handset and it takes about two minutes.
The order of those codes matters more than people expect, so it has its own page. Read it before you start.
3. Carrier voicemail goes off
This is the step people skip, and skipping it is the single most common reason a setup catches nothing. If your carrier voicemail answers a missed call first, the network sees the call as answered and nothing else runs.
You are not losing your voicemail. It moves to me instead, and you get a readable transcript rather than a beep. How to turn it off, per carrier.
4. We run a test call
Someone rings your number from a different phone and lets it ring out. I check that the call arrived, that the divert worked, and that the text-back actually went to the caller while you did not pick up.
You are not live until that test call passes. That is the definition, not a formality. Until a real missed call has been caught end to end, nothing has been proven.
One thing to check before any of this
Your mobile plan has to be postpaid. Prepaid plans cannot divert calls to an arbitrary number. On some carriers it fails outright, on others it depends on the plan, and in every case it fails quietly: the codes appear to work and nothing gets caught.
If you are on prepaid, tell me before we start rather than after.