MSS provides simple "call waiting" service. When a calling party makes a call to called party which is in busy state, MSS will hold this call and play waiting music to the calling party if the called party has 'call waiting' service right. Once the called party is available, MSS will reconnect prevous call to the called party automatically.
MSS can support 'call waiting' at SIP phone side. It is no problem. In fact, if SIP phone processes 'call waiting', MSS will not do it again.
No, you cann't at this time.