Problems with IAX Softphones over 3g/4g mobile data network

I’m having tremendous issues with audio from users that have Zoiper Softphone installed on their smart phones (iPhone and Android both). Here is the setup:

FreePBX System

FreePBX Distro (latest)
Asterisk 11.5.1
VMWare ESXi guest
2 vCPU
SIP Trunk to VoicePulse West Coast (SJC)

All of the hard endpoints (Polycom) work great for inbound and outbound calls. The softphones on mobile devices work just fine when on the internal WiFi network (I just realized I have not tested call quality on external wifi).

Call quality is TERRIBLE if the softphone is on a mobile data network - even 4g. Inbound audio is great, but sound going outbound from the softphone is choppy and often garbled to the point of incomprehensibility.

This is the same for all of the following cases if the softphone is on mobile data 3g/4g:

  • zoiper >>> PSTN
  • PSTN >>> zoiper
  • zoiper >>> internal hard endpoint (polycom)
  • polycom >>> zoiper

All of the above cases work fine if the softphone is connected to internal WiFi which indicates to me that this is clearly an issue with the mobile data network that I need to tune out.

I have tried changing codecs (gsm, ilbc, siren7 etc) to reduce the bandwidth of the call, but these have made no difference. I even went so far as to purchase and install the g729 codec from Digium (and the corresponding license on the zoiper android client).

I have also changed the IAX2 settings for bandwidth (low, medium), but calls are still so severely garbled to be unusable.

I’m at a loss now - maybe jitter buffer…(?) I left it set at the default 200 / 10000.

Thanks in advance

This may be a bit of a “basic” theory, but many carriers have a much lower upload rate than download rate, which would explain the good quality incoming audio and poor quality outgoing audio. Not sure if that’s the issue here or not, but may be worth checking into.

I can’t do much about the wireless carriers - (I think all of them are Verizon, might be an AT&T in there), but I can’t imagine I’m the only one who deals with this configuration.

For my office I actually have well tested and confirmed 30 Mbps down and 8 Mbps up (consistently).

I’ve also configured my edge device (ClearOS) with traffic shaping to prefer IAX2, SIP and RTP traffic, giving a dedicated 1Mbps (with only 4 channels - that should be WAY more than enough B/W)

I did a test with disabling jitter buffer for IAX2 channels (only the softphones are IAX) and that seems to have helped some. I also confirmed that WiFi from an offsite location (so not on the mobile carrier) works well.

This makes me think it is more of a jitter/latency problem than a straight bandwidth issue. Mobile data generally has decent enough BW for a VoIP call (even bad 3g approaches 1mbps each way) but those networks tend to be high latency and somewhat jittery - especially if you are moving and traversing cells

Its rather the opposite. I have never heard of anyone having an acceptable experience. It’s all about jitter and latency. You can’t buffer that away.

Using a VPN tunnel that uses TCP rather than UDP cab help when you have bandwidth to burn.

Follow me with confirmation and DISA is uaullay your best bet.

Bummer - but that’s the conclusion I was coming too as well.

Thanks for the help!