"Flagged as Spam" can mean a couple of different things.
Some reasons why you may see this:
The contact opted out by blocking your number but did not text STOP
Carriers have received a high number of spam reports from one of your recent messages or campaigns
The message contained a phone number different from the one you are sending from
The message contains content that the phone carrier has chosen to block because similar content was used in other spam messages on their network
Their carrier is incorrectly marking your messages as spam (this generally resolves itself).
How to resolve it?
Call the phone number and ask them to text START back to you
Resend the message without the phone number
Change the content of the message
Leave the conversation for some time and retry the message (you can schedule the message to send later so you don't forget)
We can submit a ticket to a contacts phone carrier on your behalf, to ask for clarification on the block
When sending campaigns, you should keep in mind the following to reduce the chances of being marked as spam:
Only contact numbers who have "Opted In"
Refrain from sending large messages, where possible
Avoid using marketing and other language such as "URGENT", "BUY NOW", etc.