USG50 with 2 WAN; VoIP; keep slower as failover?

I'm using the Zywall USG50 for traffic management and dual WAN capability. I have a cable internet line at 30/4 and a dsl line at 6/768k. In this environment, I use VoIP for almost all calls. I primarily have the slower DSL line as a backup, and have configured it as a failover on the USG50. However, I've read that the USG50 can be slow to detect outages (2+ minutes) and switch over. Does it make sense to incorporate the DSL line not just as a failover? My goal is for a more seamless experience for users (both internet and VoIP) should the faster cable internet line go down.