Working network that no longer works.
Jitter is the rapid change of network latency to the degree that buffering can not supply a steady audio tone. That is usually cause by large data bursts or surging in the switch and poor bandwidth control. It is not the latency per se that is causing the problem but the rapid ramping up and down that is causing it. It's like trying to drive your car by smashing the accelerator then letting off it real quick and wondering why it's hard to drive 40mph steadily.
Here's what I would look for:
- New Switches added or rewired creating a loopback and the 7400 doesn't know how to handle it
- New phone added to the 7400 that has finally broken something
- Potentially bad phone spewing across the network
- New Server/Services added to the network that are bombing things (Microsoft loves to program for higher bandwidth instead of efficiency)
- Someone's PC (or PCs) have gotten hijacked and are now spewing outdata all over the network.
The simplest test would be to take a small 8-port switch, hook up 4-5 phones to it, then run that switch directly into the 7400 (does not connect to the office switch). Test all-call paging, see if it works, make sure there are no issues, etc etc. This would eliminate basically everything listed above and give you a good referenceable starting point.
Once you have a good base-line to work from, the trial and error method means you would slowly add more "Variables" until you find out where it breaks. Does it break when you run just 4-5 phones through the office switch? Does it break when you plug in the 34th phone? Does it break when you Turn on Joe's computer? Does it break when you start the company accounting/CRM/etc package? The slow but painful process of elimination always yields pretty exact results while taking the most time to find.
The smarter option would be to start by hooking up SNMP monitoring to the switch (provided it can do that) or putting an ethernet tap between the 7400 and the switch and seeing what is happening. The problem with process of elimination is you never learn exactly WHY something did what it did just the effect it had. The old "teach a man how to fish not give him one" theory.
I'm guessing that since VLAN and/or QoS is not available on your network that the switch in question does not have that functionality.
So given that assumption, are your computers piggy-backed through the phone or does the phone and computer have their own physical line to the Switch?
Also, is it a real switch and not a HUB? Make and Model of switch would be nice.
As far as your question goes the answer is No. QoS and VLAN are not requirements of an IP Phone system. They are merely a method or "tool" to help insure that you have fewer problems within your installation. It's kind of like Backboards. Sure you can throw stuff on the wall and it may work well for years but there are some standards and practices that make it so much more easier to maintain. Same principle different application