Android added a plus sign before the number, making an undesired international call

I cannot repeat this bug, but I can see it in my call log. Last Monday I called a friend with a number beginning with 328 (Wind provider, Italy). According to the log, I called a number starting with +328 which was an international call to Belgium, and in fact some people answered me in French. I tried to repeat the bug, but without success. I only know that in some (rare) occasions the plus sign is automatically added to the call. This is causing me trouble because I don't want to pay for undesired international calls. How can I avoid this? I know that I never manually dialed the number, I always choose it from my contact widget menu.