Roger Troutman's distinct sounds are created by the synthesizer. You may think creating sounds on the synthesizers are complicated, but sounds preferred by talkboxers are simple lead sounds. Below are commonly used fundamental sounds:
One characteristic of these sounds is that they are quite simple. Lead sound presets on synthesizers are commonly associated with detune and multiple oscillators to give it a thick rich sound. These sounds could be effective to use as effects, but could also be mechanic shifting farther from the Roger Troutman sounds. In another words, high-tech synths are not necessary.
Roger Troutman used Minimoog from Moog Music during the 70s and the 80s and DX 100 from Yamaha during the 90s (he also temporarily used Yamaha's DX 7 before moving to the DX 100). Most of his famous recordings were done on the Minimoog. If you are trying to decide on a synth, you would also want to choose an analog/analog modeling synthesizer or an FM synthesizer.
Videos on the right are live performances of his classic hit “So Ruff, So Tuff” - The videos are recordings of his performances at Aruba Sinbad's Soul Festival in 1998, Summer Jam San Francisco in 1993, BET Summer Showcase in 1989, respectively from the top. He is playing the Minimoog in 1989 and DX 100 in 1993 and 1998. You could easily hear the difference between the sounds of the Minimoog and the DX 100 (of course the quality of the recordings makes a bit of a difference as well), but both are very distinctive sounds of Roger Troutman.
To achieve a Roger sound, a monophonic synth with fat high frequency and sharp low frequency is favored. You would also want to play with the modulation/pitch bend in order to emulate the Roger Troutman style sound.