I use a VOIP service called phone power
www.phonepower.com, that works out at around $10/month. It's reliable and I use it several hours per day for business. VOIP works fine over cable/broadband. It's a little trickier if you have DSL. The main problem is there is more of a delay between when you talk and the person on the other end hears you, but it's really no worse than a cell phone.