Our telephony provider has conducted an analysis of the issues that cause a partial outage of inbound telephony and provided us with a reason.
According to available data, one of their signaling IPs of the upstreams has been experiencing a DDoS attack which has been mitigated on their end. This has caused NoACK behavior, where ACK was not being received to our 200 OK from the upstream end. The service is now operational and investigation is being held on the upstream side to prevent this from re-occurring.
Apologies for any inconvenience caused.