There is no guaranteed solution to this problem because the phone number is not physically stored on all SIM-cards, or broadcasted from the network to the phone. This is especially true in some countries which requires physical address verification, with number assignment only happening afterwards. Phone number assignment happens on the network - and can be changed without changing the SIM card or device (e.g. this is how porting is supported).
You can follow the Google suggested way of picking up the number. In order to verify the taken number, you can send an sms (containing a code) to the number and control the response by using SMS retriever API mentioned here https://developers.google.com/identity/sms-retriever/overview. This way you can make sure that the number is correct and working.

There is no guaranteed solution to this problem because the phone number is not physically stored on all SIM-cards, or broadcasted from the network to the phone. This is especially true in some countries which requires physical address verification, with number assignment only happening afterwards. Phone number assignment happens on the network - and can be changed without changing the SIM card or device (e.g. this is how porting is supported).
I know it is pain, but most likely the best solution is just to ask the user to enter his/her phone number once and store it.