Not sure about keeping your current number but I think you can with magicjack
I have magicjack and it is awesome. I've had it two years. Pays for itself in the first 2 months then its like I is free. 5 years of service cost me $70.
You have to have a computer on for it to work. I have Voicemail and it even emails you a file with the message so I can play it on my phone or another computer or you can call the number and listen as well.
Call quality is good. Even with the $10 vtech phone I bought.