1. A method for training a modem comprising the steps of: determining whether characteristics of a current communication line used during a current call are substantially similar to characteristics of a prior communication line used during a plurality of prior calls; if the characteristics of said current communication line are substantially similar to the characteristics of said prior communication line and if the number of the plurality of prior calls is greater than a value, then using stored data mode information from said plurality of prior calls without using said characteristics of said current communication line to design a signal constellation for use during the current call's data mode; and if the characteristics of said current communication line are substantially similar to the characteristics of said prior communication line, and if the number of the plurality of prior calls is not greater than said value, then using stored data mode information from said plurality of prior calls and said characteristics of said current communication line to design a signal constellation for use during the current call's data mode.