Whether it is a physical sim card or an esim, the phone number remains.
You can export your iphone contacts to a VCF file. Put this file on a usb c stick drive or write yourself a mail with the VCF as an attachment, so you can easily re import the contacts when you switched to Graphene OS.
You should also disable iMessage and Facetime on your old iPhone, otherwise iMessages from your friends and family etc will not arrive on your Android phone. Background is, iMessage to your number are send as simple sms if iMessage isn't available.
But when you leave it on, they will be parked at apples message server until you use your iphone again (because the apple id is connected with your sim card).