Welcome to the community and thanks for your post.
Sorry to hear you're having difficulties getting roaming to work while abroad.
It sounds as though you've connected to the local network but are unable to make calls.
Its worth just double checking roaming is active when you log into Your Account .
Click on plan and device
Click the tab manage services
Make sure roaming is set to on here. If it isn't move the slider to on and click update. This will present you with a green success message. It may take up to 24hrs for this to go live on your account.
You can also check the call barring option to stop calls abroad isn't set to on. This maybe what's causing your issue as you mention your SIM doesn't allow roaming.
You could also try the following steps if roaming is on:
Turn your phone off and then back on again
Go into your phone settings and select a network manually.
Try removing your phone’s battery, while it’s still switched on – this helps reboot it and find a local network
If after these steps you are still experiencing difficulties give our mobile team a call on +443456000789 from a roaming handset or land line so we can take a further look into this for you.