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Area:
Messages

Summary:
When connected to the mobile network via WiFi Calling only, iMessage and FaceTime phone number activation always fails. They also do not appear to detect the correct current phone number. This was tested with multiple carriers. 

Steps to Reproduce:
These were tested with two SIMs from two different WiFi-enabled carriers (Verizon USA and MTS Russia) while in three different countries:
- Settings, Messages, disable iMessage
- Settings, FaceTime, disable FaceTime
- Optional: Settings, (Apple ID), Sign Out
- Insert SIM card from WiFi Calling-enabled carrier
- Settings, Phone, Enable WiFi calling
- Either: travel to area without mobile service and/or enable airplane mode
- Optional: Reboot phone
- Settings, Phone, confirm that "My Number" is correct
- Make test phone call to confirm connectivity
- Send test SMS text message to confirm connectivity
- Optional: Settings, sign-in to Apple ID
- Settings, Messages, enable iMessage
- Occasionally, "cannot activate iMessage" appears immediately
- Wait for "Send & Receive" list to appear (avg ~10 seconds)
- "Send & Receive" list will often show the wrong (previous) phone number
- Spinner will appear indefinitely next to (incorrect) greyed-out phone number


Another faster test, when starting from an enabled iMessage state:
- Settings, Phone, confirm WiFi Calling is enabled
- Enable airplane mode
- Confirm that phone is still connected to carrier (via WiFi Calling)
- Settings, Messages, disable iMessage
- Settings, Messages, enable iMessage
- "Waiting for activation..." appears indefinitely while on WiFi Calling

Diagnostic attempts such as waiting 24 hours, signing out/in to iCloud, rebooting, and erasing/restoring the phone do not have any effect. 

Returning to an area with mobile service usually results in quick re-activation.

Expected Results:
Successful iMessage and FaceTime activation.

Actual Results:
No iMessage/FaceTime phone number activation.
Settings - Messages - "Send & Receive" spinner appears indefinitely
Settings - Messages - "Send & Receive" list shows wrong phone number
Occasionally, an activation failure error message appears immediately. 


Version/Build:
iOS 12.1.2 (16C101)
Verizon carrier file v 35.0.2 / PRL 15599
Mobile TeleSystems carrier file v 35.0 (MTS Russia)

Configuration:
iPhone 7, 128GB