We don't know if this is just restricted to Telstra in Australia or all GSM WP7 devices, but evidently on at least some Windows Phone 7 devices, call waiting is not enabled. Odd, eh?
Over at Chris Walsh's blog, he demonstrates how to enable this seemingly important feature and it's far from difficult:
Just type *43# into the dialer, then press "Call"
Of course there is nothing unique about that code per se, as they are just part of the GSM feature codes which are universal. Anyone else confirm whether or not this is disabled on their WP7 device?
Source: Chris Walsh (blog)