Ubuntu – Simple Voip server for Ubuntu

A lot of this depends on the bandwidth that your phone system is going to use - if it's a call center with hundreds of people on the phone simultaneously you'll have a higher need for a separate network than an office of hundreds who just want phones on their desks but aren't heavy users. A conservative (as in, "way more than is really used") estimate is 128kbps per simultaneous call, or 8 calls per megabit, so gigE can do approximately 8000 simultaneous calls (okay, a little less due to overhead, but that's the right order of magnitude). So it's unlikely to be a bottleneck. And as long as you have QoS so that big downloads won't affect your phone traffic you should be fine.

Something you don't address is the Power Over Ethernet (PoE) angle - PoE switches are a significantly more expensive (both to buy and to run) than their non-PoE counterparts. One nice feature of PoE is that a single UPS on the switch keeps all the phones running during a power outage.

What I've done in the past is: run a PoE and a non-PoE port to everyone's desk. The PoE is labeled 'Phone', the non-PoE is 'data'. This preserves data speeds for the user's computer, gets the aforementioned power redundancy for the phone, and allows me to (eventually, if needed) separate out the networks more.

So I guess what I'm saying is: don't bother with 'software' non-convergence, but do bother with separate networks where the hardware matters. If the hardware's going to be the same though (ie. you're running all PoE or all-non-PoE switches) then a converged network should be fine.

At a guess, your problem isn't LAN-side but on the cable modem. If you have 10 Mbps down and 2 Mbps up, you have slightly more outbound bandwidth than with the T1 link, but you are likely having more jitter and generating more outbound traffic. I'd suggest putting QoS on the cable router (or the link to the cable router), guaranteeing a portion of the bandwidth for VoIP.

Depending on packetization interval and protocol used, you're looking at roughly 30 kbps to 90 kbps per simultaneous call.