Do you know if Spanning Tree Protocol (STP) has an adverse effect upon VoIP? If so, what is the best way to provide LAN-based switch redundancy to VoIP gateways? Be aware that classical STP convergence time is about one minute -- it is terribly slow. There is a rapid STP or RSTP which takes a few seconds but may still be too slow for VoIP.
There are other possibilities. Some vendors have special protocols to replace STP -- for example, Extreme has Ethernet Automatic Protection Switching (EAPS), which takes about 50 milliseconds.
Another alternative way to build redundancy is using L3 switching and dynamic routing; this will have the advantage of being very fast.
Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP) is fast enough or even Open Shortest Path First (OSPF), which is a little bit slower, but also good enough for VoIP -- and of course, Cisco has its own version of RSTP which they call RPVST+ and provides convergence time of less than one second after a failure.
A network with STP has one big disadvantage -- it's very hard to troubleshoot.
My suggestion is L3 switching with dynamic routing.