I was trying to buzz someone into my building today using DTMF, and it didn't work. After I set the key sounds to Level 2 (I must have previously disabled them without thinking), the tones now seem to work as expected. I am also hearing a tone sound when the key is pressed.

In the past, my other phones did have a selection for short (45ms) or long (70ms) tone duration; short tones would allow quicker navigation of voicemail system menus, etc, while long tones were sometimes necessary for certain systems that could not detect the short tones. I don't see this option available, and assume that the long tones must be in use on the N900.