Fix a regression that is introduced by RTP events generated by DSP

For those internal calls that list in extension.conf like call-forward(*21*). The Read() function
that used to get the target number could not retrieve the DTMF with the previous fix as it is not
received by asterisk core.

[sip0-direct]
exten => _*21*,1,       GoSub(sub-create_cfim,s,1(sip0))
...
[sub-create_cfim]
exten => s,1,           Read(input,,,,,300)