Can’t make calls to the eu

As far as I’m aware the APN will affect mobile data en you are abroad but not calls.

Are you currently in the UK? If so are you dialing 00 followed by the country code and dropping the 0?

if outside the UK you would need to dial the international access code for that country. Usually 00 but nit always. 

for example to dial 0666 777 7777 in France you would dual 0033 666 777 7777 (often shown as +33 666 777 7777)

Some phones (but not all) will allow you to enter “+” and automatically change this to 00 or the appropriate access code for the country you are in. 

If you are in the UK you will need to purchase an out of plan add on as standard plans only include UK calls and texts from the UK.