Cannot access Nortel Voicemail System

I am trying to use my iPhone to access voicemail systems at two offices, both running Nortel voicemail. They require you to press the * button twice on to access the voicemail system. When I press the * button, nothing happens on the other end of the system - everything else works when dialing, but the * button is not recognized. Does anyone know if there was a change in the coding for this on POTS v. the iPhone or does my iPhone have a software glitch for that button?