A dedicated internet line, if you can afford it, is always ideal imo. I think your IT guys are on the right track. Yes the cost can be high; however we use VOIP & in the year+ I've been with my company (a technology firm), the phones have never been down. I don't know if we ourselves have a dedicated internet pipe, but I know it's what we'd likely recommend to a customer in your scenario. In addition, the sound/voice quality we have is really, really good. Basically, if you trust your IT department, & they're making the recommendation, go with it. IT can be expensive, but if it is there's often a very good reason.