Currently, I'm also unable to fetch the phone number of an incoming call,

there's an Apple official Live Caller ID Lookup Extension way, and another hacky way of updating the in-app contact lookup directory everytime when the app is launched.

Live Caller ID Lookup Extension

The Live Caller ID Lookup app extension requires you to use Apple relay servers to support making calls to your server endpoints. This requires endpoint validation from Apple. If you’re interested in using the app extension, submit your request. For more information on the server-side API, see the Live Caller ID Lookup example and the Swift homomorphic encryption library.

Cons: submitting an Apple form to request access to this feature, even then we get an encrypted phone number to our server, and we need to do homomorphic encryption to all our leads to be able to search on encrypted data using homomorphic encryption