Hi,
I am in a rural area, and using fixed wireless broadband provider. I have had blueface now as my sole "landline" type service for over 2 years and find it excellent. It is highly dependent on the qualitry of your ISP provider, so will vary across different providers. I know a guy on a different wireless provider who has awful problems.
The fact that your provider is also offering voice services should however give you some confidence, as ultimetly he will be responsible for ensuring the network is good enough to carry voice, and he should be able to manage the Quality on the network to ensure it.
You could always download a trial from blueface and test it out, its free, and you wont need hardware for the trial.
Regards,
Wexfordman