The following are our recommended frequency band operators:

America: Suggest T-Mobile
Australia: Optus / Telstra /Virgin Mobile/ Vodafone
Germany: E-Plus/ O2 / T-Mobile/Vodafone
France: Bouygues Télécom/ Free /Orange /SFR
Spain: Movistar/Orange/Vodafone / Yoigo
Italy: 3 (Tre) /Telecom Italia Mobile (TIM) /Vodafone /Wind
UK: 3 (Three) /EE (Everything Everywhere) /O2 /Virgin Mobile /Vodafone

Vodafone users and other operators not on the list need to check:
https://www.worldtimezone.com/gsm.html
https://willmyphonework.net/
https://www.frequencycheck.com/