When Maedhros had—during an evening of drinking under the stars in Himlad—confided to the middle three of his brothers about his fear of being ignored after the incident at Thangorodrim, Celegorm had loudly proclaimed, "there shall never come a day where you call and I do not answer. Not as long as I yet dwell among the living. This I swear to you, brother!"
That had led to much alarm among them, oathbound as they were. Yet Curufin had repeated Celegorm's words, swearing to always respond whenever Maedhros called him. And before long, Caranthir too had sworn the same despite Maedhros' vehement dissuasion. "Oaths are not to be trifled with", he'd said.
"All the more reason for us to swear this," Caranthir had countered. "There are no insincere oaths, but false promises aplenty."
And Maedhros would deny tearing up at the underlying warmth in his brother's words. He was not a morose drunk, no matter what Curufin asserted, so it must've been the smoke from their campfire, for none of his brothers had commented as he had swiped under his eyes. Celegorm hadn't even looked at him as he'd spoken into the silence, "you were forgotten once, brother. Never again."
True to their words, ever since that night, every time Maedhros called, his brothers responded. None of his letters to them went unanswered, not even when he worded them less than affectionately amidst some ongoing disagreement. Never did it happen that he asked them something, rhetorical it may be, and his brothers stayed quiet. Even in sleep, if he called their name to check if they were awake, their fëar would reach out in response, reassuring him that they heard him.
So when, in Doriath, he called, "Curvo! Moryo! Tyelko!" and they answered not, nor did their fëar weave with his own, he knew.
He knew that there lay no beating heart inside their bloodied body. No breath left in their lungs to say, "I hear you, brother."
He knew. His calls will remain unanswered.
He knew. He shall be unheard once again. For the day had come when he called and his brothers did not answer.
He knew their oaths to him stood fulfilled. For they dwelt no longer among the living to keep it.
Maedhros knew. His brothers were dead.