It's easy enough to, for example, make a call from an iPhone app and then use the comma pause character to send DTMF signals to navigate through a tonepad menu:
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tel://somenumber,,1,,2"]];
But it doesn't seem to work if you are already in a call, and want to place a second call. When I try this from the app with the code above, it dials somenumberbut seems to ignore the ,,1,,2 completely. Can anyone confirm this?
From my testing I discovered that iOS indeed ignores any instances of ,,1 etc, once you have established a phone call with the first party.