On iPhone, go to Settings » General » Network and set 'Enable 3G' to 'OFF.' iPhone will now send short DTMF tones. You should re-enable 3G after use so that you don't stay on Edge service if you don't have to.
[crarko adds: I haven't tested this one. I'm not a phone person so I had to look up the acronyms. IVR is the recognition technology that allows voice activated dialing and other kinds of verbal commands. DTMF is basically sound modulation to use tones to transmit data, like phone numbers, improving on the old pulse dialing systems and the telegraph. OK, so I'm a bit old-fashioned.]

