I have worked with both systems and I agree with MitelInMyBlood. The Toshiba is a good system for Small Businesses that are stand alone, ie... they don't have branch offices, remote sites or remote users. The VOIP with the Toshiba has nowhere near the reliability or functionality of the Mitel and is rather behind most other VOIP systems. I have installed many Toshibas in businesses like doctor's offices, car dealerships, etc... Those places typically don't need or want the VOIP functions as they really are not needed. It is a good Cheaper system!
If you are looking for a great VOIP phone system, I really don't think that you have to look anywhere else but the Mitel product, either the 200 ICP or the 3300 ICP. These systems are very flexible in most situations and they also have embedded Voice Mail. I am pretty sure that for the Toshiba system, you would need to purchase the VMail as an extra. Actually, you will probably get a package price, so you may not know that unless told.
Good Luck.