Most of the good value accommodation offers you can find booking online the Venice Hotels, is handled by big International Companies with a system called "Allotment".
The Hotel Owner accept to sell a certain amount of his Venice Hotel´s rooms to the Company at a very low price, to get the high traffic it can provide.
When you book online you get a "Voucher" from the Company that you present to the booked Venice Hotels being sure they will honour that.
We had experiences with customers who wanted to be reassured of their booking, therefore calling the Venice Hotels by telephone.
Mainly in "high season", the Venice Hotels Owners have no interest to occupy a room in name of the International Company, so it happened that they tried to deny, at the telephone, the validity of the booking.
But again when you´ve booked Venice Hotels via credit card, they really have to accommodate you, if not in the booked Hotel, in another Venice one, of the same or better quality.
So do not ask telephonic confirm when you have a "Voucher" in hand, or if you do, clarify that you know very well your rights and Hotels obligations on the credit card booking.