iPhone won't operate my front door buzzer

The 2.1 upgrade fixed most of the touchtone issues, but I still cannot use my iphone to buzz someone into my apartment building. I've tried many other phones with no problem, but the iphone wont trigger it, all I need to do is push 9. Anyone else have this issue? Or better yet a solution? I'm getting tired of telling the UPS guy to try calling other people in the building to get in.