Ribbon SBC 504 Gateway Timeout: Zoom Phone trunk shows offline

SBC integration errorsMANUAL FIX INSIDEFREE — NO SIGNUP
Quick answer. Zoom Phone marks a trunk offline when it stops seeing a successful response to its SIP OPTIONS keep-alives; Ribbon's default keep-alive behavior behind a stateful firewall often fails them. The usual root cause is NAT: the firewall tears down the UDP/TCP pinhole between the Ribbon SBC and Zoom's signaling addresses before Zoom's next OPTIONS arrives, so the keep-alive times out and the trunk flips offline (504 on the next real INVITE). The fix is to keep the pinhole alive with a shorter SIP OPTIONS interval than the firewall's idle timeout, verify the Zoom signaling addresses are allowed, and confirm with Ribbon's own keep-alive counters.

The raw code check

Run this before you change anything — it confirms the root cause in one pass:

SBC keep-alive counters — failingFAIL
SBC> show sip options keepalive
zone: zoom-sig interval: 60s idle-to: 30s
sent: 240 ok: 0 timeout: 240
last 504 at 03:14:07 — pinhole torn at ~29s
SBC keep-alive counters — healthyOK
SBC> show sip options keepalive
zone: zoom-sig interval: 15s idle-to: 60s
sent: 240 ok: 240 timeout: 0
trunk state: online — steady for 24h

The manual fix — 3 steps

  1. Set the SIP OPTIONS interval below the firewall idle timeout. On the Ribbon SBC, set the SIP OPTIONS keep-alive interval for the Zoom signaling zone to 15 seconds (any value comfortably below the firewall's UDP idle timeout, which is commonly 30–60 seconds). A 15-second interval keeps the pinhole permanently open.
  2. Allow Zoom's signaling addresses on the edge firewall. Confirm the customer's firewall permits SIP (UDP/TCP 5060–5061, or TLS 5061) from Zoom's published signaling IP ranges to the SBC, without SIP inspection/ALG in the path. A packet capture on the firewall interface is the honest check: you should see OPTIONS out and a 200 OK back.
  3. Verify with the SBC's own counters. Run the keep-alive status command again after one full interval cycle: sent and ok should increment together with zero timeouts, and the Zoom Phone dashboard should show the trunk back online within a minute.

The automated alternative

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