After activating a device (or getting a new number on an already active device), some users may experience an issue where they can send a text to an iPhone (or other iOS devices) but can't receive texts back from them.
Most phone numbers these days are recycled. Occasionally, these phone numbers may have been used by an iPhone user who registered it with iMessage. Because of this, communication continues to attempt going over Apple's iMessage service. This causes issues since the number is now associated with a Troomi device. For example, when the parent sends a message to the Troomi device, it may be sent to the previous owner of that phone number.
How to tell if the number was registered with iMessage
When you text the number from an iPhone, the text bubble is blue and you can see it's sent using iMessage.
Otherwise, the text bubble will be green.
How you can fix it
Apple has a tool that should let you deregister the number with iMessage.
https://selfsolve.apple.com/deregister-imessage/
1. Scroll down to the bottom section: No longer have your iPhone?
2. Enter your phone number. and select Send Code
3. If you don't get a reply on your Troomi phone, open the Parent Portal
4. Check the Blocked Texts in the audit log for the 6-digit code and enter it on the site. (You might have to refresh the Parent Portal a few times until the blocked text shows up).
According to Apple's documentation on this:
After you complete deregistration, your phone number is removed from iMessage services. You should be able to receive text messages right away but it might take a few hours for some Apple devices to recognize that you're not using iMessage when they send you a message.
If this doesn't work, we can try to find you a new number that hasn't been registered with iMessage before.