edia & signaling by default go through the Asterisk box (i.e. Asterisk sits in the media path). This has advantages when it comes to the simple (but non standard) implementation of features such as music on hold or call recording. It also allows implementing features independent of the end point's (i.e. phones) capabilities using old fashioned DTMF tones for feature invocation instead of e.g. SIP signaling. Requiring media to go through the call control server has significant disadvantages as well, among them limited scalability, additional delay and jitter in the media path, as well as introducing a single point of failure. Using the SIP re-invite method SIP-to-SIP calls can sometimes be re-routed around the Asterisk box under certain conditions but only after the call first was established through the Asterisk PBX.
Media gateways: Asterisk is architected as a one-box solution including gateway PCI cards from Digium. While it can interoperate with external gateways, the easiest setup is using the Digium cards.
Hope it answers your question.
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