Attached are three VERY good reads for understanding SIP Dialing rules in CUCM and an overview of Session Initiated Protocol. 

I had to design a solution for 7960 and 7940 IP Phones where the protocol was SIP and there was no T.302 or Interdigit Timeout when phones would dial internally to another IP Phone extension.  The configuration article explains what types of phones can support KPML or Key Pad Markup Language versus which phones send the digits enblock.  The difference being that the newer phones ( Gen 2 79x1 and 79x2 ), will send their digits one at at time using KPML. 

Hope this helps someone out some day.