Why business whats-app account number showing as pending?

I have created an app on the Facebook Developer platform and set up a WhatsApp webhook for sending and receiving messages. In the WhatsApp API setup, I added two phone numbers; the first one is working fine, but the second one shows a status of "pending." Both my business and WhatsApp accounts are verified, and a payment method has been added. The only difference between these two accounts is that the number with the "pending" status does not have two-step verification enabled. I tried adding it via the Graph API, but it returned an error stating that the account is not registered. In short, I need assistance in getting my mobile number connected instead of showing "Not applicable" or "pending." Your help in this matter would be greatly appreciated.