Hello,
There are two types of situations in which the plus sign is used for international dialing instructions:
1. The way we use it on this site where the plus sign has a separating role between the elements of a calling sequence (IDD code, country code, etc). For example, to call from the US to a phone in Italy you would enter: 011 + 39 + phone number; however, you don’t actually dial these plus signs, you just use 01139 followed by the Italian number.
2. You will sometimes see an international number starting with a plus sign. The example above will be: +39 followed by the phone number. In this case the + replaces 011 which is the US IDD or exit code. Here you will actually have to dial the plus or use 011 instead – either way works.
The plus sign can be dialed when using a cell or VoIP phone to place the call. Look for + on the phone’s keyboard (for cell phones it is usually one of those characters that can be entered by dialing a button twice). Many landline phones do not have the capability to dial +.
There’s also an article we wrote a while ago on the subject:
https://www.howtocallabroad.com/qa/plus-sign.html