S5 won't make calls or receive. "Conditional forwarding".

A

AC Question

Last week my sister sent me her old S5 (I live in Spain, she in the UK). I immediately went to put my sim card in but it was too large so I carefully cut it down to size. After that data and sms were working fine, but the day after I tried to call but it wouldn't work. Gave me the message "conditional forwarding..." and then stopped the call.

Any ideas what could be causing this? The chopped down sim? Network settings or something like that? It's strange that sms and data work fine but calls and MMI codes don't.

Weirdly today the calls worked for about five minutes and then stopped, I could check my credit with the MMI code for the first time. Now it's back to not working.

Any help greatly appreciated.