In an effort to battle illegal robocalling, the FCC along with the carriers, have created a framework called STIR/SHAKEN. STIR stands for Secure Telephony Identity Revisited. SHAKEN stands for Secure Handling of Asserted information using toKENs. The industry-wide mandate is to prevent fraudsters from scamming consumers and businesses through robocalls and illegal phone number spoofing while making sure that legitimate calls reach the recipient.
Effective June 30, 2021, calls sent to most carriers must include a valid calling number. Please review your configuration to confirm you are delivering a valid outbound caller ID. All calls with invalid calling number will be denied with a SIP 503 Service unavailable error.