From Phoneware (a VoIP carrier):
Unfortunately, there is very little that a phone carrier can do to prevent calls from being identified as SPAM. SPAM, robocalls, and telemarketing have become a big problem. In their attempts to address this problem, many mobile and traditional landline phone carriers are handling the identification of SPAM calls differently. As a result, it is not possible to prevent a phone number from being inadvertently labeled as SPAM. It is possible for an individual to tag calls from a phone number as SPAM in error, or out of spite, or even malice, which can result in carriers or calling platforms identifying the telephone number as SPAM for all calls and to all of their subscribers. For example, if someone uses Verizon as their mobile carrier and has a an Android phone, when they receive a call from an unknown number, they may easily report that call as SPAM with just a press of a button. But this does not only identify the phone number as SPAM to this subscriber, but is transmitted back to the carrier to increase the likelihood of being seen as SPAM to ALL Verizon subscribers. There are also layers of SPAM handling at the OS level, i.e. Apple iOS or Google Android, which can also complicate the issue. If someone is informed that their phone number is showing up as SPAM, it may be possible to report this misinformation to the carrier that incorrectly tagged the call as a SPAM call, and to request that they remove the SPAM label from the phone number. Here are a couple of web articles that highlight this topic: http://www.uyt.co/2020/10/28/what-to-do-if-your-phone-number-is-marked-as-spam/
Here are websites for mobile carriers to report errors in SPAM reporting:
T-mobile - https://callreporting.t-mobile.com/
Verizon - https://www.voicespamfeedback.com/vsf/
There are some recent legal protocols and procedures that have been implemented with the goal of reducing unwanted SPAM and robocalls. But these new methods of blocking robocalls will not automatically remove the incorrect tagging of phone numbers as potential SPAM numbers by mobile carriers and platforms. This article explains the new procedures being implemented by carriers and required by the FCC: https://en.wikipedia.org/wiki/STIR/SHAKEN
We hope this information helps provide understanding of why calls from your phone number(s) might be incorrectly labeled as SPAM to callers, and what can be done to correct it by working directly with the carriers who are incorrectly identifying your calls as SPAM.