Hi ​@Yas2014 

Yes, you can use your phone in Belgium with iD Mobile. Belgium is included in their European roaming zone (Band 1), meaning your standard UK allowances for calls, texts, and data will work at no extra cost, subject to a fair-use policy.

To ensure everything works smoothly when you arrive, follow this quick checklist:

Check Your Plan: You have access to your regular UK allowances, but data is subject to a fair use cap of up to 30GB per month.
Enable Roaming on Your Phone: Go to your device's settings and turn Data Roaming ON.
Activate Roaming on Your iD Account: You must toggle Roaming ON in the iD Mobile App or via your online account dashboard.
Manual Network Selection: Connect to an iD Mobile partner network upon landing, go to your network settings and manually select the one that supports customers of iD Mobile
https://community.idmobile.co.uk/roaming%2D76/which%2Droaming%2Ddestinations%2Dare%2Dincluded%2Din%2Dmy%2Dplan%2D45112

Please be aware that if you are near the coast and you have your connection set to ‘Automatic’ then it may connect to a maritime (satellite) signal and this will be costly.
https://www.idmobile.co.uk/help-and-support/roaming

You can also set a cap for your monthly bill that ensures that if your phone inadvertently connects to a ‘payable’ network then it won’t work (as you won’t have the £ to pay for it):-
https://www.idmobile.co.uk/help-and-support/capping

Hope this helps x