The voice path between MM and the PABX can occur in two ways - a station (either analog or digital) i.e. an NEC would use analog extensions, a Nortel would use 2616 digital sets. Or you can use trunks, E1/T1 QSIG, IP H.323 or SIP.
This is the bearer for voice. Along with this there is signalling that tells the MM how to handle to call. For example, E1/T1 and H.323 use QSIG to send messages about the call, as well as MWI on/off requests. SIP uses RFC compliant messages and the station method is different again, either an RS 232 link, or via reading displays of the digital sets once a call is answered.
Regradless of the method used, as stated before, stations (VoIP or TDM) dial an access number that send calls to the trunks/hunt groups. When MWI requests are sent, these go to the PABX for processing, so wether its an IP or TDM device, the PABX knows where it is registered and manages the requests(s)