You have a Teams Phone deployment that uses Direct Routing on a single Session Border Controller (SBC). Users report that outbound PSTN calls are failing, but inbound PSTN calls are successful. When y

Question

Options

• AThe online PSTN gateway is disabled.
• BAn SBC certificate is expired.
• CAn external DNS entry is missing from the FQDN of the SBC.
• DThe phone numbers of the users are defined by running the New-

• A
  52% (14)
• B
  7% (2)
• C
  30% (8)
• D
  11% (3)

Explanation

In Direct Routing, Microsoft sends SIP OPTIONS heartbeats to the SBC to monitor its health and confirm it is reachable before routing outbound calls through it. When the online PSTN gateway (the SBC pairing in Teams) is disabled via the Teams admin center or PowerShell (Set-CsOnlinePSTNGateway -Enabled $false), Microsoft stops sending SIP OPTIONS to that SBC - exactly matching the described symptom. The SBC can still send SIP OPTIONS to Microsoft (which Microsoft will respond to), so the SBC appears healthy from its own perspective. However, because Microsoft considers the gateway disabled, it will not route outbound calls through it, causing outbound failures. Inbound calls can still succeed because the SBC initiates those independently. An expired certificate (B) or missing DNS entry (C) would typically break both directions.

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