The number you bought is not the number you give out. Your customers keep calling the number on the van, the website and every invoice you have ever sent. Nothing about that changes.
What changes is one setting at your phone company: your existing line forwards its calls to the new number, and the assistant picks them up.
The setup almost everybody wants
Your phone rings as it always has. If nobody answers after about four rings, the call goes to the assistant instead of to voicemail. You lose nothing, and nobody reaches a beep.
From a mobile line, dial this once from the phone you want to forward — the exact code with your number in it is shown in Settings → Phone number:
*61<your assistant number>**25#
The 25 is seconds, roughly four rings. Use 20 for three, 30 for five. Under 15 you barely get a chance to answer; over 30 the caller has already hung up.
To turn it off: `##61#`
Other ways to forward
Landlines and VoIP
Those codes are for mobile lines. Landlines and VoIP differ by carrier and there is no single answer. Call your carrier and say you want unanswered calls forwarded to another number — it is a standard request and takes a minute.
One thing worth testing
Some carriers pass the original caller's number through a forward and some replace it with your own. If it is replaced, the assistant cannot recognise who is calling and will ask for their name instead. Call your own line once from another phone and see which one you have.