Exam MS-721 topic 1 question 2 discussion

Actual exam question from Microsoft's MS-721

Question #: 2
Topic #: 1

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 you review the SBC logs, you discover that Microsoft is responding to SIP option requests, but is NOT sending SIP option requests.
What is the cause of the issue?

A. The online PSTN gateway is disabled.
B. An SBC certificate is expired.
C. An external DNS entry is missing from the FQDN of the SBC.
D. The phone numbers of the users are defined by running the New-csTeamsUnassignedNumberTreatment cmdlet.

Show Suggested Answer

Suggested Answer: C 🗳️

by Vlado123 at Sept. 11, 2023, 7:11 a.m.

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Shamzee

3 weeks, 4 days ago

Selected Answer: C

Issue with resolving FQDN for the SBC for outbound call

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shah2

6 months ago

Selected Answer: C

The issue you're describing is most likely caused by an external DNS entry missing from the FQDN of the SBC (Option C). When the external DNS entry is missing, Microsoft Teams cannot properly resolve the SBC's FQDN, which can lead to issues with SIP options and call routing

upvoted 2 times

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54b197b

6 months, 3 weeks ago

Selected Answer: C

If the SBC is disabled, how is it sending SIP option requests? However, a failure to send SIP option back to the SBC could be for lack of a (or incorrect) FQDN to send them to.

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734acc2

10 months ago

If Microsoft is responding to SIP OPTIONS requests but not initiating them, it indicates that the connection from the SBC to Microsoft Teams is alive, but there may be a problem with how the SBC is configured to handle outbound calls.

upvoted 1 times

54b197b

6 months, 3 weeks ago

Correct, in this case the incorrect configuration is likely FQDN related

upvoted 1 times

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Vlado123

1 year, 9 months ago

Correct https://learn.microsoft.com/en-us/microsoftteams/troubleshoot/phone-system/direct-routing/issues-with-outbound-calls#no-users-are-able-to-make-calls

upvoted 4 times

54b197b

6 months, 3 weeks ago

That link answers no calls being made but that is not the question. In the question, some calls are working while others are not. Further, the question focuses on MS not sending SIP options. This indicates network issues such as firewall, FDQN, etc.

upvoted 2 times

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