Its something called Contact link-communication and it can be seen in other series too
From Gundamofficial:
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Originally Posted by Gundamofficial
An alternative means of short-range voice communication. When mobile suits come into direct physical contact with each other, or with other vehicles, sound can be transmitted between them via vibrations in their outer armor. A mobile suit can reach out with its manipulator to establish the link, and these sound vibrations can also be relayed through wires, including the wire-guns used by human workers. Unlike radio communications, conversations transmitted in this way are almost impossible to intercept.
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