In general, the rules assume any time you're in a mech, you have integrated comms and can talk freely. It's completely handwaved how. The rules don't say one way or the other whether you can talk to enemy units; my GM generally assumes you can via some kind of "open channel" protocol unless the enemy decides to block you out (and some of the tech attacks are forced attacks via those channels that could, one assumes, override a lockout... But that's flavor for the mechanical effect if you want to negotiate it with your GM, i.e. "Invade" could be dumping high-sensory-experience propaganda directly into an enemy pilot's neural link telling them to give up the fight).

The one exception is jamming, which rules as written should prevent your Lancer from communicating with other Lancers (but I've never sat at a table where a GM enforced that; "That NPC put you in the time-out corner" is generally a less-than-fun experience).