Can anyone help me a VoIP Question

Ian Price
Ian Price used Ask the Experts™
Hi Experts

I am on a personal mission to under a little bit more about VoIP.

So we run a Gamma \ Horizon system at work. All configured and working via Cisco.

Our Wifi is controlled via Unfi USG, Switches etc. I've plugged in (With Consent) one of our spare Voip phones into a spare port on one of the Unifi switches and configure on it own Vlan etc.

The phone can call external numbers no problem, but if I try to ring an internal ext the internal phone rings but once connected I can not hear the person on the other end.

I get that something is blocking it or its not configure I just do know where to start trouble shooting,

Maybe someone could point me in the right direction?

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David FavorFractional CTO
Distinguished Expert 2018

Problem might be some sort of high port blocking.

Each VOIP system tends to work in a slightly unique way, so likely you can see problem quickly running a packet sniffer like tshark, looking to whatever port block is causing the problem.
David FavorFractional CTO
Distinguished Expert 2018

Tip: One technique you can use to potentially narrow the problem area is disable all firewalls every where in your VOIP packet flow.

Then enable each device's firewall, one by one, till calls stop working.

This approach generally gives clues which might speed up fixing the problem... as working with packet sniffers tends to be tedious...
nociSoftware Engineer
Distinguished Expert 2018

Voip is using several protocols to get to work.
1) SIP - Signaling, it makes the phones ring (on INVITE message) or announce their availability (on REGISTER messages).
2) RTP - Real Time protocol for the "Media" channels. (The Voice, one channel for each direction).
(and possibly a few more, these are most commonly used)...
All are UDP based (SIP can be TCP, but is mostly UDP).

SIP is used to negotiate a conversation, including portnumbers on the endpoints. And there is the problem... VOIP has one enemy NAT.
NAT transforms IP address & Ports..  In your case the outgoing voice is handled OK, the voice from ext -> int is blocked by the NAT Router (firewall)

Some routers come equipped with ALG Application Layer Gateways. For VOIP they have been mostly implemented those WRONG... so they cause more trouble than they solve.
In General: DISABLE SIP ALG.  (modern PBX can cope with Far end NAT quite well).

You may need to enable the firewall rules to allow UDP in the range of the phones for receiving them. In the case of NAT you may actualy need to port forward them.
(An On prem. PBX can help as well so you can forward all traffic to the on Prem. PBX and use that as proxy in between all conversations).

For Toying around:
3CX has a "home use" PBX that you can use free for a few phones.
FusionPBX can be an option.
Both need a VM  or dedicated hardware to run.

For seeing what is negotiated sngrep  is an excelent easy to use tool that can be used to monitor traffic onany system where PBX traffic passes (PBX or Firewall based on linux).
If you can take recordings in the form of pcap files then wireshark can help decoding the traffic and if you also have the RTP to listen to the sound channels.
Distinguished Expert 2017
Issue is your VLAN to VLAN traffic.
i.e. when you connecting through the PBX out you are fine, but when you are trying to dial an internal Extensions it sounds that the VLAN on which this phone is does not match the VLAN on which the other phones are. Is there a direct path from the WIFI vlan on which this phone is to the VLAN/IP range where the other phones are?

While all the points provided are valid consideration, the issue is that you have one path cofirmed to work so I think those constraints and consideration might not apply as they may have caused the same issue when dialing out from this phone

Your wifi phone <=> VLAN <=> wifi <=>   router/FW <=>Internet
                                          VOICE SW <=>    PBX   /    
    Local Ext <=>  VLAN VOIP  /
Ian PriceIT Manager


Arnold, I think this make sense to be honest.

The office phones are on VLAN 216 which is setup and controlled via Cisco Switch\Router that I do not have access to. I presumes the either the switch or router issue DHCP address.

However some on the phones are not on a VLAN at all as they have the same ip numbers as our computer picked up via our Windows server DHCP and they seem to work fine. I am guessing this is an oversight and should be like that.

So I wonder is I try putting two phones on the same VLAN if that will solve the problem.

I'll try after the holidays

Thank you all for commenting

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