You do not need a new number
This is the part most people get wrong before they start. Setting up an answering service does not mean changing your business number, porting anything, or touching your phone system. You divert unanswered calls to it, and everything else stays exactly as it is.
Turn your voicemail off first
This is the single most common reason a divert appears to do nothing. If MessageBank or your carrier voicemail is active, it answers the call before your divert has a chance to fire. Turn it off in your carrier app before you set anything else up.
The no-answer divert code
On an Australian mobile, no-answer diversion uses a standard GSM code. You dial it once on your handset and press call. The number at the end is the ring delay in seconds before it diverts, and twenty seconds is a sensible default.
Telstra, Optus and Vodafone
All three use the same GSM codes on mobile, so the process is identical whichever you are on. Landlines differ by carrier and usually use a short sequence starting with a star or hash, which your provider can confirm.
If you are on a phone system
A business running a hosted PBX or a VoIP platform such as 3CX, Aircall or RingCentral does not use handset codes. The divert is configured in the provider's admin panel instead. It is still straightforward if you have the login, and considerably less so if it sits with an IT provider.
Turning it off again
Cancelling a divert is a similar code and takes about the same time. Your phone goes back to behaving exactly as it did before, which is worth knowing if you are nervous about trying it.