One common reason may be an algorithm used by a carrier or a 3rd party anti-scam app detects a high volume of calls being made by the number.
Another common cause is the number had complaints submitted against it by call recipients. These reports can be done erroneously. For example, some users might report a number simply because they don’t like being called by a number they don’t recognize.
The quickest way to overcome this issue is to use a different valid Caller ID number.
If that solution is not practical, you may be able to remove your number from these lists by contacting the carriers that marked it directly.
These links will allow you to potentially remove a Spam label. see links below for the 3 major US cellular carriers.