Why is there a plus symbol in front of some phone calls that come in and not in front of others

[Android Central Question]

I'm curious as to why when some calls come to me on the cell phone from a cell phone there is a plus symbol in front of the number as it appears on the caller ID. Dial other incoming phone calls do not show the plus symbol. I checked to see if it is only something as simple as a cell phone or not a cell phone calling. That's not the case does this mean that there is a third party that is also privy to these conversations or text messages?