Assuming my original interpretation, whilst the help for microsip is not at all clear, it looks like you have to enable media buttons when configuring it, and have a suitable device:
Handle Media Buttons
Enables handling of media keys or buttons events on multimedia keyboards or headsets with buttons (WM_APPCOMMAND message). Can be used for call answer, hold, resume and end call.
In that case, it is a microsip question, not an Asterisk one.
In case someone would like to go deeper to your suggestion
sendrecv - Used to establish a 2-way media stream.
recvonly - The SIP endpoint would only receive (listen mode) and not send media.
sendonly - The SIP endpoint would only send and not receive media.
inactive - The SIP endpoint would neither send nor receive media