Call Quality - During Call Volume Dims to Barely Audible then Comes Back

[loyer]

I have an office with 10 or so Yealink T48S behind and SBC that is having an odd call quality issue. I am pretty sure it is isolated to their office but I can't figure out what is causing it.

This is what is happening.....
1) caller in office makes an outbound call to me on a non-3cx phone line.
2) we are talking just fine, very clear, sounds good....
3) Then I can barely hear his voice... it is as if he has is hand over the receiver
4) He can hear me perfectly --- I can't really understand his words but can hear that he is talking.
5) After maybe 10 to 20 seconds, I can hear him again perfectly.
6) This might happen two or three times in a 5 to 10 minute phone call.

He is not on speaker phone. He is calling from a new (purchased with the last month) Yealink T48S.

Any ideas?
Thank you for any help.

 

[leejor]

Does this ever happen on set to set calls, behind the SBC?
Are there sets at another location (same as server), if so does it happen to calls there?
Is there more than one trunk group for outgoing calls, if so, does it happen on all trunk groups?
What type Codecs are you using? Is it one that the trunk group supports, or is there transcoding?
Did this just begin? If so, was anything at all changed or upgraded?

 

[loyer]

Does this ever happen on set to set calls, behind the SBC?
---- I am not sure, I will have to ask

Are there sets at another location (same as server), if so does it happen to calls there?
---- There are two other locations (smaller only 2 to 3 people) and it is not happening there.

Is there more than one trunk group for outgoing calls, if so, does it happen on all trunk groups?
---- Only one trunk for outgoing calls. Twilio

What type Codecs are you using? Is it one that the trunk group supports, or is there transcoding?
---- For the Twilio trunk, it is setup to use G.711 U-law.
---- For the Yealink Phones, they are provisioned to use (in order top to bottom): PCMU, PCMA, G722, G729

Did this just begin? If so, was anything at all changed or upgraded?
---- This is the first that I am hearing about it but we just put this office in place in the last week. It is hard to say because we had the phones installed in the office for a while but the phone number just got ported over last week. Also, last weekend, we upgraded the 3CX PBX to SP3 ... but that may just be coincidence.

In the SBC logs, I am seeing a bunch of these:

ERR | 20180215-141912.508 | 3CXTunnel | TUNL | 3560 | CallSession.cpp:327 | Current Offer/Answer state is Terminated; expected - Idle

But I think that is could multiple people trying to answer the same call at the same time... not sure.


Thank you for any help you can provide.

 

[leejor]

---- There are two other locations (smaller only 2 to 3 people) and it is not happening there.

Are these other sites also behind SBCs? Do calls to these locations , from the site having issues experience the same audio problems?

Trying to narrow down if it might be the SBC, at problem site, or possibly a network provider issue. If no other site has a problem, with outgoing calls, it pretty much eliminates the server end, along with the outgoing trunks themselves. That leaves a possible hardware/network issue at the site having the problem. If you have a spare SBC, you might do a swap, or try registering a set, at that location that does not use the SBC, see if it has an issue.

 

[loyer]

Are these other sites also behind SBCs?
---- One is and one isn't. The one that isn't is a single user. The one that does have an SBC is two users. They said they aren't have any issues.

Do calls to these locations, from the site having issues experience the same audio problems?
---- Yes, if the good locations calls the bad location, there are issues. (and I am pretty sure same for bad calling good).

I also believe it may be something specific to the location having problems. The only reason I am not 100% sure is because this location has 10 users so I keep wondering if it is some sort of capacity issue. I have set everything I know to do in the firewall (Zyxel Zywall 5) even though the SBC makes it so you shouldn't have to.

We are also occasionally getting a few dropped calls at that location.

If there were problems with SBC, would that make the audio volume drop so much? Seems odd.

Thank you again leejor for all your help

 

[leejor]

Usually network issues don't cause low volume , you'll get things like clicks and pops, or missing bits of words, in the case of dropped packets. Latency will usually cause a delay, or echo type problems. Low level, without any other problems, would suggest that the packets are getting through. Almost an analogue issue. Wireshark may provider you with some answers. As I suggested, try a STUN set without the SBC, see if it has the same problem. Consider swapping out the router, if you have a spare one handy, on a test basis. Bypass any switches on the local network.

Have you tried using a 3CX softphone on the network?

Are the sets, the same make/model as the other locations? Same firmware version?

 

[loyer]

Just an update....

I was able to mostly resolve this issue by doing a few things:

- provisioning phones --- first factory resetting them, then reprovision
- turning off encryption on the SBC
- adjusting the host windows computer settings:

-- the windows os was also the domain controller and had DNS server running. It was using 10K+ UDP ports. I adjusted the settings of the DNS server to lower the number of ports it used.

-- the windows os had some unused network cards and IPv6 enable... it disabled all unused NICs and turned off IPv6

-- I double/triple checked that on both firewalls all types of ALG or any sort of traffic analysis, IDP, filtering of any kind were turned off.


After doing all of those things late one night, when the users came back into work the next day, everything worked significantly better.

I don't know which of those changes fixed the issue (or if they all did) but finally got it working good enough.

Thanks leejor and everyone else that helped me figure this out.

 

[leejor]

Unfortunately, when you want to get a problem solved ASAP, you don't have the luxury to test out fixes one by one. Which leads to not being certain, what actually did the trick.