IP Phone SIP Calls Dropping or Delayed – Need Help Diagnosing

[Pringlec]

Hi everyone,
I'm currently troubleshooting a recurring issue with Zoom Phone/Poly IP phones where SIP calls are either delayed or dropped intermittently.

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Details:

• Phones: Poly CCX 500 and Edge E300 models

• Call path: SIP calls via Zoom Phone

• Some calls go straight to voicemail or drop without audio

• Other times, there's a delay before the callee hears ringing

• My own test calls to the same numbers go through fine

• Problem seems intermittent and inconsistent

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What I've Tried:

• Enabled logging on the phones (set to Debug)

• Captured traffic using SPAN on switch ports

• Collected packet captures on both sides (caller & callee)

• Noticed TLS sessions but traffic is encrypted (can't inspect payload)

• Confirmed phones are updated and network is stable

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Ask: Has anyone run into this issue before? Any tips for:

• Decrypting or analyzing encrypted SIP/TLS traffic?

• Zoom Phone-specific call signaling insights?

• Additional logs or settings I should look into on Poly devices?

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Happy to provide more details if needed — I’m just trying to figure out where the issue is happening (Zoom, network, or phones). Appreciate any guidance!

[sparrow]

What debug level modules did you enabled on the phone? Have you engaged your firewall team to capture SIP Logs on their side? 

[Pringlec]

Thanks for the reply

For the debug logging, I set the Global Log Limit to Debug, and for the Module Log Level Limits, I specifically enabled Debug for the following modules:

Application

DNS

Network

SIP

SRTP

These modules seemed most relevant to the signaling and media handling issues we've been seeing.

As for the firewall side, yes — We have started capturing logs and packet data on the firewall. I have looked though our firewall settings and I do not see anything explicity blocking anything from zoom. We are also allowing traffic from all the ports they mention in their KBs. 

Its weird becacue this issue kind of came out of no where where as about almost a month ago everything was working fine and now all of a sudden there are tons of reported issues. 

We also have two other locations and the other two location have identical firewall rules and there is no reported issues with Zoom Phone.  

[sparrow]

Are you running BYOC or you using Zoom as your PSTN access? Are you having the same issue with the Zoom app?

[Pringlec]

Wea are using Zoom as PSTN. We do not have issues with Zoom App. 

[Pringlec]

We are using Zoom as our PSTN and we do not have issues with Zoom App its primarly issues with the poly phones 

[Eliot]

hi @Pringlec 

new hp/poly zoom edition firmware for ccx and edge e series dated 3/16/2025 addressed a number of issues.  I have an e450 that was intermittently losing network connection.  I did a factory reset which automatically updated the firmware.  No more issues.

Current as of 4/26/2025 poly/zoom ccx firmware is 9.0.1.6562

Current as of 4/26/2025 poly/zoom edge e series firmware is 8.2.3.0870

Zoom Phone Certified Hardware

Did my response answer your question? If so, please don't forget to mark the reply as an accepted solution.

thanks,  eliot