One of the most challenging aspects of supporting a VoIP phone system, is the fact that we are completely dependent on the customer’s network to operate properly.  If there are no branch offices or telecommuters and the phone service comes in over conventional phone lines, then the phone system is only dependent on the local area network.  If there are branch offices or telecommuters, or if the phone service is delivered by SIP trunks, then the phone system also has to deal with the customer’s Internet connection and firewall.  Therefore, it is very important for the customer to have in-house staff or IT consultants that are qualified to support their network.


Local Area Network


The customer’s local area network usually has no problems but it certainly isn’t the case that it never has problems!  The most common problem is having more than one device on the network with the same IP address.  If the customer has decided to have dynamically assigned addresses for their phones, then they must ensure that their DHCP server isn’t handing out duplicate addresses.  We usually end up diagnosing these networking problems simply because when a person’s phone isn’t working, it is very noticeable!


Wide Area Network


If you have branch offices or telecommuters on the phone system, then everyone’s Internet connection and firewalls come into play in the operation of the phone system.  Management of customers’ firewalls is not part of our support offering.  We are familiar with some but certainly not all the routers we encounter.  We can specify the port forwards and other settings that the phone system needs but we must rely on the customer to make the configuration changes on their routers.  If the customer is using MPLS or has VPNs established between locations, then it usually all just works, especially when a service provider is managing a MPLS network.


SIP Trunking


If you are getting your phone service across the Internet via SIP trunks, then the quality of your Internet connection and the configuration or your router makes all the difference between perfect calls and “choppy” calls.  When we are exhibiting at trade shows, often visitors to the booth will tell us stories of having had a hosted PBX service with terrible call quality.  Being a PBX vendor, we usually come to the defense of their PBX service!   We’re not just being defensive.  It is absolutely true that choppy calls on a hosted PBX service are caused by an inadequate Internet connection and not by any problem with the PBX.


Most people focus on the speed of their connection but that’s not the only factor involved in good Internet telephony.  Latency is the measure of delay in the network and that determines how quickly your spoken voice is heard by the party on the other end of the call.  If the round trip is more than 300 milliseconds, then the delay will be obvious and the parties will often “talk over” something the other party is saying.  Jitter is a measure of how variable the latency is in the network.  The endpoint receiving voice packets must buffer them up and possibly get them in the right order before it paces out the audio to the listener at the right speed to faithfully sound like the speaker.  With high jitter, some packets might arrive too late and the IP phone or other VoIP device will discard the late packets and pick back up at the right spot in the sequencing of the packets being played out to the listener.  This will cause audible dropouts in the conversation.  Finally, packet loss, where some of the packets just don’t make it to their destination will obviously result in dropouts in the conversation.


The other crucial factor in successful SIP trunking is the router that will be receiving the voice packets from the phone service provider.  If the Internet connection is being shared with other data from the Internet, then the router must support “Quality of Service” and it must be properly configured.  This feature allows the small voice packets to be sent out ahead of other, possibly larger data packets.  This gives the time critical voice packets priority over the other data packets.  To avoid this problem completely, you can have a dedicated Internet connection for the SIP trunks and the voice packets don’t have to compete with anything.  Finally, the router just needs to be good enough for the job.  We recently had a customer who upgraded to a synchronous 100 Mbps fiber Internet connection and the quality of their calls dropped dramatically.  After much investigation, it was finally determined that the router was simple having a hard time drinking from that fire hose!  The router was replaced with one that was adequate for the connection and all was well.


For more information on sizing and configuring a network for Voice over IP, see this article on our Documentation page Planning Networks for VoIP .