Summary
Before moving your number to Sidephone, turn off RCS (Android) and iMessage (iPhone) to avoid issues with text deliverability. After disabling, wait a few minutes and send yourself a test SMS from another phone to confirm.
Note: It is recommended to disable RCS and iMessage before moving to Sidephone to prevent missing text messages. While it can be done afterwards via Apple and Google's website, turning it off in settings before removing the SIM card is more reliable.
Steps
If you are coming from Android (turn off RCS)
Open Messages on your Android phone
Tap the three dots (⋮) → Settings
Tap Chats (or RCS chats)
Turn RCS chats OFF
Restart your phone
If you no longer have your Android phone or the steps above cannot be completed:
Go to Google’s “Disable Chat” page in a browser: https://messages.google.com/disable-chat
Enter your phone number and confirm with the code sent by SMS
If you are coming from iPhone (turn off iMessage)
Open Settings on your iPhone
Tap Messages
Turn iMessage OFF
Go back to Settings → FaceTime and turn FaceTime OFF (recommended)
Restart your iPhone
If you no longer have your Android phone or the steps above cannot be completed:
Go to Apple's "Deregister iMessage" page in a browser: https://selfsolve.apple.com/deregister-imessage/
Enter your phone number and confirm with the code sent by SMS
Verify after disabling
Insert your SIM into Sidephone (or another non-smartphone)
Ask someone to send you a regular SMS (green bubble / standard text)
Confirm the message arrives on Sidephone
Considerations
You already ported your number: Still disable RCS/iMessage to prevent delayed texts.
Texts work for some people but not others: This usually means RCS/iMessage is still partially active, repeat the steps.
Android → iPhone or iPhone → Android switch: Disabling both services is safest.
No access to old phone: Use Google’s “Disable Chat” page for RCS; iMessage cannot be disabled remotely without Apple support.
When to Contact Support
Create a support ticket if you cannot receive standard SMS on Sidephone after disabling RCS/iMessage, restarting your old phone, and re-inserting the SIM on Sidephone.
What to Collect for Support
Were you previously on Android or iPhone?
Is RCS toggled off in Android Messages (or disabled via Google’s site)?
Is iMessage turned off in iPhone Settings?
Can you receive a plain SMS (not Wi-Fi message) on Sidephone?
Are messages failing from all contacts or only some?