On VOIP you need the same ammount of bandwith on both directions.
56 kb modems are only 56 kb from the net to the customer, On the direction from customer to the net they are, at their best, 33kb.That is done that way because 99% of users need more bandwith from net to user than user to net.
I do not know of anyone using dial up for a gateway. I see that it could be done with ISDN, since it is a digital signal and you could use a multiple ammount of ISDN circuits to get more bandwith.
Some video conference units combine up to eight channels of bandwith to create a good “pipe” for a dial-up video conference.