The official WhatsApp platform needs a number that is not currently registered on the WhatsApp app or the WhatsApp Business app. A number can only be in one place.
The easy option: a new SIM
A prepaid SIM for ₹100 to ₹200. You need it once, for the verification code. After that it can sit in a drawer; nothing runs on the phone. Keep the SIM active with a small yearly recharge so the number is not reassigned.
Using your existing business number
Possible, if that number is on the WhatsApp Business app and not your personal WhatsApp. We back up the chats, remove the number from the app and register it on the platform, on a call with you. From then on you read and reply through the assistant and your own personal number.
Never use your personal number
You need your own WhatsApp to receive alerts, take over chats and read the daily summary. The business number and your personal number must be different.