Illustrative example. Dana, her family and every number below are invented, assembled from the kind of mishap that lands on my paper list. No real person or company is meant. The point is what the numbers did, and what fixed them.
The contact was labelled "Mom", and for six years it had never failed. It held ten digits with dashes in the middle, the way Dana's mother had written them on the back of an envelope: 212-555-0100. At home, in New York, that was all a phone needed. Nothing about it looked wrong, which is exactly why nothing about it ever got checked.
A week in Lisbon, told in order
Dana flew to Portugal on a Sunday night for a conference. She landed, her phone found a local network, and a friendly message arrived from her carrier about roaming. She had asked her carrier about international calling and roaming in advance and had it in writing, which is the part of this story that went right. On Monday evening, after a long day, she tapped "Mom" to say she had arrived safely, and the call did not go through. What a phone does with a bare ten-digit number abroad differs by phone and by network, and in her case she got a recording that made no sense to her. She tried again and heard the same thing. A few years earlier she would have blamed the signal. This time she tried her brother.
Her brother was saved as 1-206-555-0100, the domestic long-distance form with the 1 at the front and no plus. That one did not connect either. The 1 is the trunk prefix of the North American plan, and it belongs to calls inside the US and Canada. It is a different thing from the country code, which looks the same on paper and sits after a plus. A number saved that way is correct only for a caller who is standing in the right country, and Dana was not.
On Tuesday she needed to reach a friend in London, saved long ago as +44 020 7946 0000. This one had a plus, so it felt safe. But the friend had written the number the way British people write it for each other, with the domestic zero in front of the area code, and Dana had copied it in exactly as she had seen it. A UK number such as 020 7946 0000 becomes +44 20 7946 0000 once the country code takes the place of the zero. With both in place the call failed, or reached an unhelpful recording, and she spent ten minutes convincing herself that the friend had changed numbers. The friend had not. The fix was deleting one zero.
By Wednesday she had a fourth problem, and it was not about calling at all. A colleague from the conference texted her, a message from a number not in her phone, and her phone, set to silence unknown callers, had sent the call that came before it straight to voicemail. Dana had that colleague's number somewhere, typed in without a country code. A phone matches an incoming call to a contact by comparing numbers, and a number saved in a different form may not be matched, so the contact she had was not recognised as the person calling. The setting itself was doing its job. On an iPhone it lives at Settings > Phone > Silence Unknown Callers, and it sends numbers that are not in your contacts to voicemail. It only helps when the contact list is tidy.
Dana's last surprise arrived on Thursday. Months earlier she had moved her contacts from an old phone through a spreadsheet, and the spreadsheet had treated each leading plus as the start of a formula and quietly removed it. Every number from that import now began with a country code and no plus. Nothing in the display showed the loss. She found it only when a call from a saved contact showed up as a bare string of digits instead of a name.
The half hour that repaired the list
On Friday morning, in a hotel breakfast room, Dana sat down with her phone and did the sort of boring repair I recommend to everyone. She did not start with a clean-up tool. Duplicate contacts are common, and merging removes the wrong version as easily as the right one, so she backed up her contacts first. She then fixed the numbers she was most likely to need from abroad before anything else, and looked for four patterns: a bare ten-digit number, a 1 followed by ten digits, a leading zero, and a number in brackets.
On the iPhone she opened the Phone app, chose a contact and tapped Edit. She tapped the phone number field, deleted any 1 or 011 at the front, and typed the plus sign by touching and holding the 0 on the keypad. Then she entered the country code, then the number, and tapped Done. For her mother that gave +1 212 555 0100. For her brother it gave +1 206 555 0100. For her London friend it gave +44 20 7946 0000, with one zero removed. Apple's menus change between versions, so she checked Apple Support when her screen looked different from what she remembered. A friend of hers on Android did the same job in the Contacts app: open the contact, tap Edit, replace what is there with the plus form, save. On many keypads the plus is under the 0 key or in a symbols row, and the labels vary because makers customise the app, so Google Support covers the Google Contacts version. The steps are close in other versions.
The format she was typing is the one phone standards use, which the ITU calls E.164: a plus, a country code and the national number, in at most fifteen digits. It is what her phone sends to the network, and it is why a contact saved that way works from any country. The pieces and the reasons are in why numbers start with +1, and the list of codes she needed for her London and Lisbon contacts sits on the calling codes cheat sheet.
The spreadsheet problem took the longest. She opened a few contacts abroad and checked that the plus was still present, and for the ones that were not, she re-entered the number by hand. Later she learned that a column of numbers should be formatted as text before an export, because a spreadsheet will strip a plus otherwise. Sync was the other trap. When contacts are shared with a cloud account, a correction on one device usually spreads to the others, which is helpful when you fix a mistake and annoying when you make one. So she changed a handful of contacts first and watched what happened before she did the rest.
What to take from it
None of Dana's trouble was mysterious, and none of it was bad luck. Each failure had a plain cause: no country code, a dialed 1 in the wrong place, a domestic zero, a plus lost in an import, and a list too untidy for the unknown-caller setting to help. All five come from numbers saved in a way that depended on where the phone happened to be. The remedy is one rule, applied once per contact: plus, country code, the number without a domestic zero or a leading 1. Do it straight away for new contacts, even a neighbour, and fix the old ones in order of how likely you are to need them away from home.
A few smaller habits would have saved her time. First, when she receives a number from someone abroad, she should check the first digits against the formats that work for US numbers and make sure a chat app or a web form has not mangled it. Second, a shop, a clinic or a school that she may need to call from another city should carry its area code in the contact, even though she reaches it with seven digits at home. A number that needs an extension can be saved with a pause or wait character, which the phone dials after the main number connects. The key for it differs between phones, so Apple Support and Google Support describe it. Third, for a family list that several people edit, agree on one style, because a mix of styles is how duplicates and failed calls begin. Fourth, she should check that work contacts and personal contacts live in the places she expects, since a number saved in a work account may not appear on another phone. A half hour with the list once a year keeps it useful, and a printed card in a wallet with a relative, the carrier and the bank in the plus form covers the day the phone is lost.
A related question, what to do when a call arrives from a number you do not know, is the business of the prefix reader and the page on calling cards and apps. For Dana, the first call she placed after the repair was to her mother, and it connected on the first ring.
Next in the bookCountry calling codes cheat sheet: reading the first digits of a number