Bandwidth Reservation vs QOS

Whhich bandwidth are you concerned about - outgoing or incoming? If you do not have control over Service Provider's router, you can't achieve true QoS(when you can make sure certain kinds of traffic get more priority than others) for incoming traffic. What you can do though, is to limit certain traffic on your router, ensuring that the remaining portion is available to the target network. For example, you can limit traffic(or certain kinds of traffic, like streaming videos) on LAN2 and LAN3 interfaces which will guarantee that LAN1 always has 3mbits available.

In Cisco terminology this is called traffic policing(as opposed to shaping for true QoS) This document is a good start : http://www.ict-partner.net/en/US/docs/ios/12_2/qos/configuration/guide/qcfpolsh.html#wpxref40342

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.