What does the +1 mean in front of a phone number for contact?

It means the phone number is expressed in an International dialing format.

The plus sign “+” means, “dial internationally, using whatever code is required, ‘in the country where I am right now’, to dial to another country”.

The “1” means you are dialing to the USA, “1” is the country code for the USA in the context of the global phone system.

The probably you will have 2 or 3 digits implying the state you are calling to in the US or, maybe some country like any of those mentioned here which are part of the North American Numbering Plan

Last but not least the phone number of the person you are trying to call.

I’ll refer you to more information about international dialing, using an answer I wrote a while ago, but it is valid and works precisely as a complement answer to this one:

T4A2A. When it comes to phone numbers, every country has a code - except in North America. Phone numbers for many parts of North America (20 countries) use the North American Numbering Plan (NANP) and share “1” as the country code.

When writing phone numbers, a “+” in the first position means the next one to three digits denote the country code. Therefore, “+1” means that the phone number belongs to NANP. In other words, the phone number is somewhere in North America.

Technically, the + is a shorthand abbreviation that says “dial the International Direct Dialing (IDD) code for your region.”

If you're in a particular country and you're planning to call someone in the same country, it is, generally, not necessary to dial the country code. Because NANP encompasses such a large geographic area, many people in NANP don't give it a thought, and don't commonly express the country code when writing down or typing phone numbers, which is, probably, why the “+1” was confusing.

Now, I'm sure you noticed that no telephone keypad has a “+” anywhere on it. So, how on Earth, do you dial the IDD? In NANP countries, dialing “011” is the equivalent of dialing the + sign.

Other areas outside of NANP have their own IDD code.

I hope this helps.

The telephony numbering structure is a hierarchy where there is an international code for any given country, and then a number plan inside that country that the regulatory body of that country is responsible for.

The number plan is called E.164 by the ITU definition and the +1 is a Country Code. After this comes a National Destination Code followed by the Subscriber Number. Inside the country you don’t have to dial the CC. In fact in many countries you don’t even nee to dial the NDC if you and the destination are inside the same area.

The international regulation in force is this one:

+1 is indeed a bit complex as this is a CC shared between several sovereign countries - basically all of North America.

This is the list of assigned Country Codes:

So getting back to the question;

+1 is the formal full way of expressing a phone number. You can skip it inside the US, but if you bring a mobile phone outside of North America it’s needed. If you try to dial 555–1234 without the preceding +1 then you will end up in an error tone (invalid number) or you reach somebody local with that number. I hence always add al numbers with the CC just to be sure…

To be honest, I haven't paid much attention to +1 for phone numbers.

I knew it had something to do with international dialing.

After looking into this, this definition provides a clear example of +1 dialing is:

For example, number +1 702-555-1212 appears on your caller id as a missed call on your UK based mobile phone.

Just add 00, which is the code used to send calls out of the UK, in front of the number to call it back. It would look like 00 1 702-555-1212 .

So the next time you're at a party and want to impress people, ask them if they know about the +1 in their call log.

Footnotes