Welcome to the Community, Nuno987 !
It is very frustrating when roaming doesn’t work at all abroad.
First, please check if data roaming and VoLTE are enabled in your phone’s settings. Then, switch the network selection from automatic to manual. Test the Italian partner networks TIM, Vodafone, WINDTRE, and ILIAD one after the other. As far as I know, TIM is the preferred partner for VoLTE, but you better test all networks. If necessary, also switch your phone off completely, briefly remove the SIM card (if available), and restart the device to establish a completely new connection.
If you are using a 5G phone, try switching the network mode in your mobile settings to 4G (LTE). I believe Sunrise’s 5G roaming network in Italy runs exclusively on Telecom Italia (TIM). If their 5G towers are overloaded or frequencies are blocked in your area, the connection may drop. Switching to 4G often stabilizes the signal immediately.
Since you are online via Wi-Fi, if all these tips do not help you can also use the chat function on the Sunrise support page if you continue to experience problems. Alternatively, you can reach the roaming hotline from abroad at +41 58 777 01 01 (Mon.–Sat. 8:00 a.m.–10:00 p.m., Sun. 9:00 a.m.–10:00 p.m.) using a different phone or your hotel’s landline.
Regards cosmo