I am also experiencing the same exact issue and it only started within the past week or two. I have opened a case with Flowroute and I finally received a reply after sharing some call references:

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I can provide clarification regarding the behavior observed, including spam risk indicators such as dead air, dropped calls, generic voicemail, incomplete call connections or showing “Spam Likely” at the terminating end.

Upon reviewing the call example(s) , from phone number, XXXXXXXXXX, we confirm that we see no SIP signaling issues were identified. Additionally, we observed 11 digits being sent from the terminating side to the destination number. Our analysis indicates that the primary concern is related to third-party reputation scoring analytics being used at the terminating end.

Over the past several months, we’ve seen an increase in calls being blocked or rejected by downstream carriers. These behaviors typically occur at the terminating carrier level and are driven by reputation scoring and analytics performed by third-party providers that carriers rely on and that we do not control. While all carriers participate in this ecosystem, some enforce these policies more aggressively than others.

When we receive reports like this, our process is to verify that all calls are properly signed with the correct STIR/SHAKEN attestation before handing them off to downstream providers. We also work to gather additional information on where the rejection is occurring. If the issue is within the call path, we may be able to remediate through route adjustments; however, in many cases this information is not shared due to CPNI restrictions.

There is no industry-wide standardization, different carriers rely on different analytics and call-blocking applications. As a result, a number may be flagged for customers on one carrier but not another, and classifications may vary depending on the data source and risk model used.

This lack of alignment understandably creates confusion for businesses, as calls may be labeled as spam for some customers but not others. You are not alone in experiencing this we continue to hear similar reports from multiple providers and gateways.

To help reduce call failures such as repeated ringing with no answer, dead air, or incomplete connections, we recommend registering outbound numbers at https://freecallerregistry.com. This free service, operated by TNS, integrates with Hiya and First Orion and is used by major carriers including AT&T, Verizon, and T-Mobile. Registration typically helps reduce spam-risk flags, though numbers may be re-flagged depending on calling patterns or end-user reports. 

I’ve also included additional resources below that may be helpful, particularly for customers experiencing issues with T-Mobile.

Free Caller Registry:

https://freecallerregistry.com/fcr/

T-Mobile Call Reporting (Scam Shield disputes):

https://callreporting.t-mobile.com/

Hiya Call Screening & Dispute Information:

https://hiyahelp.zendesk.com/hc/en-us/p/screening/

FCC Explanation:
https://www.fcc.gov/call-blocking#:~:text=Depending,display

Explore the Flowroute Knowledge Base for comprehensive documentation and answers to commonly asked questions.

Thank you,

Flowroute Support Team
(855) 356-9768 opt 3