25142118: iMessage on iOS should support multiple phone numbers

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Description

Summary: With the increased availability of unlocked phones (including through Apple's own iPhone Upgrade Program), users are becoming less beholden to their home country carriers when traveling internationally.

iMessage on iPhone only support the device's current phone number, causing most users' primary iMessage ID to be unstable when the SIM is swtiched. This is despite the fact that having users contact other users from a foreign, unknown number caused a lot of confusion. More importantly, iMessages sent to the US number go undelivered without clear indication as to why.

Steps to Reproduce:

1. Give your contacts your phone number as the primary means of communicating with you.
2. Travel internationally and use a local SIM card.
3. Send iMessages to your contacts.
4. Note that your contacts are confused by your phone number.
5. Have attempt to message your home country phone number.
6. Note that delivery fails (or falls back to SMS on a temporarily unavailable number).

Expected Results: That I am presented with the option to use my home country phone number even while my iPhone's SIM is tied to a different number.

For example, when WhatsApp detects that the device's phone number is different from the one that is registered with the service, it asks users whether they want to switch the number. Declining to switch allows users to maintain a stable network ID even when the user's phone number changes.

Actual Results: Users' primary iMessage ID switches to the device's phone number.

Version: iOS 9.3

Notes:

Configuration: iPhone 6s (AT&T in the US, various carriers abroad)

Attachments:

Product Version: 9.3 Created: 2016-03-14T14:19:08.266820 Originated: 2015-03-14T00:00:00 Open Radar Link: http://www.openradar.me/25142118

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Modified: 2016-03-14T14:19:08.267110

[openradar-mirror]

Modified: 2016-03-14T14:19:08.267110