Avoiding charges while outside EU

I am currently on holiday outside the EU and I forgot to cancel voicemail or disable my Smarty SIM before I left, and I was billed a few pounds for mobile data before I realised my mistake. Own fault, but I would like to make sure I avoid any more.

I have now added a local SIM and turned the Smarty one off but I think I read somewhere that I should disable voicemail otherwise any messages will be chargable incoming calls. Can I do that somehow without incurring additional charges in the process?