at a valid openzone point you should get the first time a box asking for your mobile number.
If not then the openzone you are connecting to is not part of the deal and is either a bt business broadband customers wifi or a bt home broadband customer.
Discussed at lenght in the iphone tech threads.
If you get a replacement iphone after sucessfull phone number registration you need to reset the iphones mac address how to do that is in my links
iPhone 11 Pro 256gb on unlimited data
iPad Pro 12.9” 2020 256gb refresh o2 family discount
Apple Watch series 4
My first mobile was in 1995 a CM-R111 from sony on Cellnet.
Wincanton South Somerset (Full 4g 3G 2g indoor coverage) Remember we are all customers here not customer services