Address books rot quietly. Every mail app saves the addresses you write to, every phone adds its own copies, and an old Google export brings the same people back under slightly different names. Ours had 675 cards. After cleaning it held 207 contacts, and they now sync between an iPhone and an Android phone through a standard contacts server (CardDAV) at our email provider, without Google or iCloud.
1. Back up first, and at every stage
Export everything before you change anything, and again after each step. On an iPhone you need no app for this: Contacts → Lists → press and hold the list → Export. Keep each export with a date in its name. You will need one of them; we did.
2. Gather every source into one place
Bring in the old exports and the contacts that only live on a phone. Most will already be there: of 159 contacts from an old Google export, 151 matched cards we had, and 110 of 113 phone-only contacts did too. The rest are the ones worth adding.
3. Remove junk by what's on the card, never by group
An automatically saved address looks the same every time: no name (or the name is just the email address), no phone number and no organisation. 100 of our cards were like that, and those are safe to remove. A card with a phone number or a company name was saved by a person, even if it has no name.
Here is the mistake. Our mail provider keeps an "Autosaved" group, and it looked like the junk list. It isn't: it holds every address ever captured, including people later given phone numbers and job titles. Deleting its members took the address book from 519 cards to 84. Everything came back from the backup, but it took two passes, because 22 cards reported success on the first pass without actually being saved. Check what is inside a group, a label or a folder, and count it, before you delete by it.
4. Merge duplicates, with a guard
Cards that share an email address or a phone number are usually the same person. Merge them onto the fullest card and keep every email and phone from the others; that took our duplicate groups from 48 to 3. The guard: a shared address such as info@ or office@ links different people. When the names on a shared email or phone don't agree, a person decides, not the tool.
5. Curate by hand
No tool knows whom you'll call again. We went through the remainder by hand and removed 312 contacts that had only an email address and that nobody needed.
6. Connect the devices, in the right order
Clear a phone's own contacts before you add the sync account, never after. Once the account is attached, a bulk delete on the phone can reach the server and remove the contacts everywhere.
Things that surprised us
- A server can answer "saved" and not save. After any bulk upload, read the contacts back and check.
- Each card has an internal ID, and the same person can survive under a different one. Comparing by ID over-counts what was lost; compare by name, email and phone instead.
- After a restore, look for duplicates again, and only remove a copy that adds nothing. One of seven we checked held a phone number the other card lacked.
Our free contacts cleaner does steps 3 and 4 in your browser, using these same rules.