@Rahul Patil: Is it saying 'Unknown number' for SIM2 in Settings > About phone, as well as Settings > SIM manager? Do you still have access to your network features for the affected SIM i.e. Calls, texts, mobile data?
Please note that SIM cards have a temporary number assigned to them before a personal number is added by your network. On occasion, this number will show up in Settings > About phone instead of your phone number, or it will say 'Unknown' where the number should be populated. This is simply a read error, and you should find that your network capabilities are unaffected, and you can still be reached on your network-assigned number. If you are concerned, then I recommend reaching out to your network provider to request a SIM swap, to see if this resolves the issue.
