This is not an iphone or 3G problem. THIS IS A NORTEL PROBLEM because I've had this problem for two years on my Motorola RAZR. Only a moron would require two identical tones (not too fast, not too short, and not too far apart) to log into a voicemail system. Heck, those idiots (Nortel) do not even follow the traditional voicemail commands (i.e., "7" to delete, "9" to save, etc...).
Hopefully Apple or someone will come up with a solution because Nortel refuses to. Needless to say, I have not bought or recommended Nortel in years.