@Rita-G It theoretically will work on all providers..... However it doesn't support VoLTE (Voice over Long term evolution) (yet) which some networks use. In the UK it works on O2, EE, 3 and Vodaphone (plus others that piggy back off these networks).
It is also dependent on the device as well, i.e some devices are 2G and 3G only which may cause an issue with network providers removing 3g service.