If I replace the SIM card in my Verizon Galaxy s4 with a pre-paid pay-as-you-go UK SIM card, then my phone will have a new UK number.

If I now want to use Whatsapp, do I have to migrate Whatsapp to the UK phone number and back again upon return?

Do I then have to tell my contacts what my UK phone number is? (by email or voice phone call, I suppose). Can I simply send them a Whatsapp text from the new number, and their Whatsapp will accept it?

What's commonly done when traveling? Use Verizon's Global plan, and pay their rates?

  • Global plan is not worldwide. Just don't do Verizon overseas. Read this cautionary tale: shankman.com/… – Mindwin Jul 30 '14 at 20:12
  • Yeah, well the UK is OK for Verizon. But their $25/*Mega*byte data charge (after 100 MB allotment) is scary. Throw in $1.29 per minute for voice... all for the low low price of $30 to set it up. I don't want to be worried about charges. I already have a Giffgaff SIM, 17.50 pounds sterling (about $25) for more communication than I could ever use in ten days. The main concern is getting Whatsapp going without having to tell all my friends a new number. – garyp Jul 30 '14 at 21:07

Switch the SIM, answer no when whatsapp asks you to switch numbers.

REF: https://apple.stackexchange.com/a/122021

If you switch SIM AFTER installing WhatsApp, you are good to go with the old number. The servers "save" your activation phone number during that initial SMS.

I tested it on my own phone (it's a Gingerbread p1000n - All hail the venerable P1000) with a borrowed SIM card, the app just kept sending messages with my old phone number. WhatsApp did not ask to switch numbers. There is a "change number" setting.

If you reinstall the app while switching SIM, then you have to restart and get a new number.

About what is usually done when travelling, I can say from the World Cup visitors we received, several just bought a new temporary SIM card after arriving here in Brazil. The local phone companies even set up special and attractive prepaid plans for the football fans.

YMMV anyway, and you should check your home carrier overseas fees and the local fares. Usually you can get a better deal buying a trowaway SIM. Here in Brazil we even do it when traveling to another state.

If you want to keep your number and still enjoy the reduced local mobile internet fees, you can try and get any old android device that can do tethering (or buy a unlocked mobile hotspot), and then insert the other SIM into that device, then tether the internet to your main phone using the old phone as a mobile hotspot. Just turn off data and use wi-fi.

On Verizon's Global (not really global) plan, I urge you to read what Peter Shankman wrote of his personal experiences with that. My advice, is: DON'T. Or if you do, monitor closely your fees.

  • That's the answer I hoped I would get ... but now I'm wondering: how does Whatsapp know how to find me if I have a new SIM and phone number? Is it like Skype in the sense that you are logged in, and Whatsapp knows who and where you are? I'm guessing (I certainly don't know!) that an SMS message uses the phone network to track you down via phone number, i.e., is tied directly to the phone number. Am I making myself clear? :) – garyp Jul 30 '14 at 20:56
  • Check the permissions for the APP: play.google.com/store/apps/details?id=com.whatsapp -- basically it can read the SIM and look up your own phone number. It uses the SMS to check if you are spoofing the phone number, but it always knows (and checks) every time you run it what is your phone number. And it uses almost all the nice permissions. – Mindwin Jul 31 '14 at 12:24
  • Your comment confuses me again. If it reads the SIM and checks my phone number every time, and I have a new number (due to a PAYG SIM card), it won't associate me (represented by my old phone number) with my new temporary number (read from the SIM). – garyp Jul 31 '14 at 22:03
  • From what I've researched, WhatsApp asks if you want to switch numbers. Also I am not looking at the app source to see WHEN it checks your phone number. Just that it CAN do it because of its permissions. – Mindwin Aug 1 '14 at 14:45
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    Tested it here with my devices, I switched SIMs and sent message to a regular contact (one I already had a conversation going with). The receiver had the new message from the same phone number from before. YMMV – Mindwin Aug 1 '14 at 14:59

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