It's been a while since I last used a 3com ImpactIQ, but I'll give this a try.
3com has utilities that you can run in Windows to modify the config parameters of the ImpactIQ - What I did (and maybe this is what is required) was to set up the device itself for multilink PPP (and then set it for either dynamic channel allocation or set it to two-channel-always).
Obviously the above does no good if you do not have the Windows configuration utility - I don't remember if it has a web-based configuration as well or if it has some sort of console ability. You may be able to set those via dipswitches on it.
At any rate, I believe you need to set up the modem itself first for the appropriate settings you want and then just feed your linuxdialing script the phone # once -- at which point the modem will do the rest by dialing up multichannel.
Mike