If your Internet needs are business critical (you lose biz or capability when it's down), then you NEED a service with SLAs. Do not get residential service and hope for the best.
BTW, if your phone is biz critical, bite the bullet and get a POTS line from your local telco. Don't play around with IP telephony unless you have an in house expert. Booze, Allen, and Hamilton's example might be good to follow (they don't use VOIP because telephones HAVE to work and work well).
Chris