My son borrowed my iPhone while getting his android swapped out under warranty. When he went back to the android we discovered something quite concerning. ALL iPhone users who had previously recognized his phone as an iMessage capable phone, continued to act as if it were still an iPhone and tried to send iMessage instead of SMS messages. If the phone was inadvertently set to “not” send as SMS (if iMessage not available), then it would never send the message. If the setting were set to Send as SMS, then it “might” eventually make it but not every time.
Apple’s solution is to turn off iMessage BEFORE you switch back (3 days if possible). This is fine for those who actually have the phone and actually know that this is even necessary.
I our case we did turn the phone back on (and fortunately it used the same SIM size of this would not be possible), and turned off iMessage. 12 hours later and my iPhone still recognizes his number as an iPhone. My only option is to turn off iMessage. For other users who do not have any of this data, it will likely just not get sent.
Per Apple it can take up to 24 hours for the deactivation to fully bake. I’ll update at 24 hours.