What is the longest possible telephone number (including international dialling code)?
Is there a standard length internationally? If not, what country has the longest?
Apparently the ITU recommendation E.164 limits telephone numbers to 15 digits, including the international prefix, which can be up to 3 digits. That does not include the international access code, e.g. 00 from most countries, 011 from North America, 0011 from Australia, and also four digits from some other countries such as Brazil and Bolivia.
So I assume that if you called a maximum-length number, internationally, from Australia, you’d have to dial 19 digits in total.
In fact, looking at this Wikipedia page, Finland has some 5-digit international access codes, 99500 and 99588, so if you dial using these then it could be 20 digits.
As for whether there are any 15-digit phone numbers, I don’t know. The only country I can think of that has longer numbers than the North American 1-xxx-xxx-xxxx (11 digits) is Greece, which has the international prefix, then 10-digit local numbers, with no area code, making 12 digits in total, i.e. 30-xxxxx xxxxx.
Edit: France also uses 10-digit local numbers, but these all start with a zero, which is dropped when you call internationally, so 01 xx xx xx xx becomes 33-1 xx xx xx xx, making the total lenght only 11 digits again.
If you look at international dialling codes, the international prefix is longest from a particular network in Venezuela, and the longest inward-bound dialling code is… Guantanamo.
So if you’re using Veninfotel Comunicaciones to call Gitmo, you dial 01990 - 5399 - and that’s nine digits already.
Not knowing anyone in Guantanamo myself, I can’t be sure if the phone numbers are built on the US model, which is ten digits, or indeed if the “5399” contributes in any way to those digits, but you’re looking at a potential upper limit of nineteen digits.
On a less extreme example, if I dial my friend in Hong Kong, I dial 00-852-XXXX XXXX which is thirteen digits. If someone from Venezuela were using Veninfotel Comunicaciones to dial him, they would be dialling 16 digits.
Actually I suspect that Germany might be a contender. There is no standard length for either the area code or the customer number there, and Wikipedia says “The default length for newly assigned numbers (area code without 0 + subscriber number) is 10 or 11 digits”. That equates to 12 or 13 digits when you include the international code, 49.
As an example, this company lists a German contact number that has 13 digits including the international code: +49 (0) 2361 937 2710 (the zero in brackets is dropped when dialling internationally).
So to dial that number from Finland, using the 5-digit international access prefix I mentioned above, you would have to dial:
99500 49 2361 937 2710, making 18 digits in all.
Osama Bin Laden’s old satellite phone number, +873 682 505 331, by the way, only has 12 digits 
"Salaam aleikum. I’m sorry, I can’t come to the phone right now. I am either on another call or hiding in a cave.
“Press 1 to join the Holy Jihad; if you are already a memer of the Holy Jihad and with to discuss your application, press 2; press 3 to drive the infidel hordes from the Holy Lands; press 4 to recreate the Caliphate in the country of your choosing. Your call is important to us, Inshallah.”
That reminds me of a “Simpsons” episode when Homer called 911 and got an automated system.
Frantically, he starts wailing on the keypad…
“You have selected…Regicide…If you know the name of the King or Queen being murdered, press one.”
Inmarsat and Iridium numbers are typically 15 digits long
Can you give an example? The satphone number I quoted above is an Inmarsat number (+873) and is only 12 digits.
Also, according to Wikipedia, “Iridium phone numbers all start with +8816 or +8817 (which is like the country code for a virtual country) and the 8-digit phone number”, making a total of 12 digits.
Here’s some examples:
http://www.soest.hawaii.edu/STAG/ssf/about/ship_contact.html
There needs to be a country code added in some circumstances. I’m trying to construct a possible ship to Guantanamo number, but I’m heading out for a bit.
I’ve heard that Avagadro’s number is pretty long.
Hmm, if I am reading that page correctly, the maximum number length is still 12 digits, excluding the international access code. The three-digit “ocean region codes” are only appended to nine-digit Inmarsat numbers. It does indeed make 15 digits if you include the international access code 011 (from the USA), but the German example I gave would be 16 digits if you included this.
By the way, I’ve Googled for some Guantanamo numbers, and they don’t seem to be very long at all, only four or five digits after the “country code”:
Columbia College, Guantanamo Bay: tel +5399 75555
Naval Station Dive Locker: tel +5399 3200 and +5399 4444
Traffic Management Office (from a PDF) : tel +5399 4206.
Right, but you can’t just dial the number by itself. You still have to dial the international access code - 011 in the US or 00 in the UK.
Right, but they’re still not longer than the longest German numbers, which can have 13 digits excluding the international access code. So unless anyone can find any counter-examples, I reckon the answers to the OP are:
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Theoretically, 15 digits plus an international access code which can be up to 5 digits, making a total of 20.
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No, there is no standard length. Germany appears to have the longest telephone numbers currently in use, with 13 digits plus an international access code, thus requiring up to 18 digits when dialling from certain countries.
When dialling within Germany, you have to dial the initial zero of the area code, so German domestic numbers can be 12 digits long. The example I gave would be dialled as 02361 937 2710. I don’t know of any longer domestic numbers, either. North American numbers have the form 1-xxx-xxx-xxxx, and UK numbers are mostly 01xxx-xxxxxx or 011x-xxx-xxxx or 02x-xxxx-xxxx, all for 11 digits. (There are a few shorter numbers in the UK, but no longer ones).
In fact I think I have found some even longer German numbers. Looking at the Wikipedia page on German area codes, there are quite a few 5-digit area codes (not counting the initial zero) in the 03… range.
Googling on a few of these came up with this page with a whole bunch of phone numbers in the town of Kleinmachnow with seven digits after the code.
With the 49 prefix for Germany, that makes 14 digits, e.g. +49 33203 8773041.
The SDMB… helping me spend my time on pointless Google research since 2002 
It reminds me of the episode where he wants to call a company in Japan (because a box of Japanese detergent has a logo that looks like his face), and asks the librarian if he can use the phone at the front desk. She says yes, if it’s local. Then he dials about 25 digits.
These aren’t full telephone lines, though, but numbers of extensions in the Kleinmachnow town administration’s PBX system (apparently in this town of 18,650 souls the town’s administration has 4-digit extensions.) The switchboard is +49 33203 877 0, i.e. just 11 digits.
There was a local flap some years ago when the University of Tübingen instituted 5-digit extension numbers (+49 7071 29 xxxxx, i.e. 13 digits including the country code). Researchers complained that their extensions could not be reached from some foreign countries, so 13 digits seem to have not been supported all over the world, at least a few years ago.
Fairly mundane, but still 14 digits: UK to Ireland. 00353 + nine digits.
Isn’t the leading zero dropped off the nine digits when calling with the international dialing code (it’s been a while).