[Freeswitch-dev] skypiax and (inbound) DTMF problem

When I used the command originate to call a skype user, say "bob", originate skypiax/skypiax2/bob 1001 I could receive the DTMF digits that bob clicked through Skype dialpad. However, when I use the same command to call a PSTN number, originate skypiax/skypiax2/+19991234567 1001 I did not receive any DTMF digits that the callee pressed on his/her telephone key pad. Why?