QoS for voice should work if applied in the correct place, which is at the bottleneck. So if using a separate modem the QoS should either be at he modem on the DSL interface, or your upstream router needs to apply outbound shaping so that it is always the bottleneck and nothing gets dropped by the modem. We have had voice running over 256K satellite services which are effectively maxed out all the time, but QoS ensures that voice is unaffected as long as it stays within its assigned bandwidth, and equally that corporate data didn't get affected by Internet access.
I may be too paranoid, after all I'm pretty sure I used to play Xbox Live with Voice Chat while maxing out my connection with Usenet downloads. Its just that when that day comes I move my telephone number over (likely a consideration once I get FTTP), AFAIK there is no going back. So if I end up on a crap service, I'm stuck.
But as my mum uses the landline explicitly to avoid the chance of a dropped call due to running out of credit, of VoIP wasn't completely solid, it wouldn't achieve that goal. We both suffer from stress especially when waiting on hold, so getting cut off is a huge issue.
I do have to say I used BT Voice a few times before they killed it, as well as O2s Voice over WiFi app before they killed that, both seemed to work well. I do keep saying to my mum, why not just keep her phone topped up with £20 or so, not much chance of burning through that in a single call, but that would be a lot to lose if she failed to hang up the call again.
Ironically, I do still have two VoIP accounts I signed up to ages ago, but I stopped using them as I couldn't find a good app for my phone. I had them for my domains, back when you werent allowed to hide your registration details.