Protocols and Endpoints
Signaling versus media
Keep the split clear: signaling sets up and tears down; media carries voice and video. SIP, SCCP, MGCP, and H.323 are signaling; RTP carries media and RTCP reports quality. A question mentioning one-way audio is almost never about signaling - it is about the media path, codec mismatch, MTP/transcoder placement, or a firewall.
Endpoint bring-up
An IP phone boots through power (PoE), DHCP (option 150 for TFTP), configuration file download over TFTP, then registration to CUCM using SCCP or SIP. If a phone gets an address but never registers, the suspects are DHCP options, TFTP reachability, or a security/certificate mismatch - not the dial plan.
SIP details that show up
Know early offer versus delayed offer: delayed offer means the INVITE carries no SDP and the offer arrives in the 200 OK; early offer carries SDP in the INVITE. Know SDP contents (m= lines, codec list, RTP port, direction attributes) and DTMF methods: RFC 2833/4733 RTP events (preferred), SIP INFO, KPML used by SCCP endpoints through an MTP, and in-band audio (least reliable).
Media resources
MTP terminates or converts media (needed for early offer, RSVP, or DTMF interworking); transcoders convert codecs; annunciators play tones and messages; conference bridges mix streams. Place them via MRGs and MRGLs into regions that make sense for the call path.
Worked example
A branch phone through a SIP trunk gets no ringback and no DTMF. Check in order: codec offered in SDP, DTMF method mismatch on the trunk, then whether an MTP is required for early offer. Set the trunk to RFC 4733 and confirm the region allows the negotiated codec.