Why wont siri use wifi? Put your iphone in airplane mode and then turn on wifi. 

Try to ask siri what one plus one is.

She will fail with the error voice dialing is unavaible during airplane mode
I can't reproduce your problem:



Maybe Siri thinks you're trying to voice dial.

Actually, maybe you don't have Siri on, as voice dialing appears to be enabled by default when you turn Siri Off.