This might sound like such a dumb question, but I upgraded my phone last September to a new phone and new brand and model (from Samsung to Sony). I kept my old Samsung just to have an extra phone which I use for work purposes. But I have noticed when I add a new contact to my old phone it will appear in my new one and vice versa.

Do you know how this could be? They have different numbers and obviously different SIMS. I've noticed on WhatsApp that the new contact appears in one phone after being added to the other one.

This is very odd, a bit like quantum entanglement.

Your contacts are updated and stored similarly to the icloud on iphone.

But I don't have icloud or that other one 'dropbox' I think. So does that mean if I sell my old Samsung galaxy if I add a new contact to my new phone it could appear still on the Samsung with the person who buys it?

Google sync?

What I was thinking too.

It will also update them from/to your gmail on your computer if you allow it.

Are they syncing across the phone, or just in whatsapp?

If in I the phone, it could be many things. If you have set up gmail, phones can also store contacts in google contacts that will then sync between phones connected to the same account. I do think on my iPhone so I never lose any contacts.

I assume you 'log in' to the phone when you set it up after receiving it using some email account or something? If you sell the phone then no because the new owner would log into their own account and not yours

As others have said, it's to do with Google. There's a crazy amount of synchronizing going on these days between apps. Recently, someone gave me their phone number so I could add them on WhatsApp. I didn't create a new contact for them in my phone book (that's done automatically by WhatsApp), but as soon as I added them they appeared on my Skype as well. I then did my monthly check on Facebook and sure enough, they were there in my friends list too.

It's very handy when I get a new phone and don't need to think about transferring my contacts over. Previously I would have to save all my contacts from my old phone to the SIM (which was limited to a certain number of entries) and then copy them to the new phone. Now I just sign into my Google account and everything's synced - contacts, calendar, photos, cloud storage. I don't know how we managed without it. Just be sure to restore your old phone to factory settings before you sell it.

I should add that it's understandable if you didn't even realise you had a Google account. Google is everywhere. If you have a YouTube account - that's Google too. Gmail is pretty obvious, but if you've ever downloaded an app or game through Play Store (and you have) you'll have a Google account.