diff --git a/packages/server/src/routes/federation.callStart.test.ts b/packages/server/src/routes/federation.callStart.test.ts
new file mode 100644
index 00000000..80e2b080
--- /dev/null
+++ b/packages/server/src/routes/federation.callStart.test.ts
@@ -0,0 +1,108 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+
+setWorkerId(1);
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let testDb: TestDb;
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  schema,
+}));
+
+vi.mock('../utils/federationAuth.js', async () => {
+  const actual = await vi.importActual<typeof import('../utils/federationAuth.js')>(
+    '../utils/federationAuth.js',
+  );
+  return {
+    ...actual,
+    getOurOrigin: () => 'https://local.example',
+  };
+});
+
+function applyMigrations(db: Database.Database): void {
+  const migrationsDir = path.resolve(__dirname, '../../drizzle');
+  const files = fs.readdirSync(migrationsDir).filter(f => f.endsWith('.sql')).sort();
+  for (const f of files) {
+    const sql = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sql.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+async function importSUT() {
+  return await import('./federation.js');
+}
+
+async function importManager() {
+  const mod = await import('../ws/handler.js');
+  return mod.connectionManager;
+}
+
+let sqlite: Database.Database;
+
+describe('processRelayEvents → processDmCallStartEvent', () => {
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+
+    const cm = await importManager();
+    for (const [fedId] of cm.getAllFederatedCalls()) cm.clearFederatedCall(fedId);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    sqlite.close();
+  });
+
+  it('Path B: zero matches → undeliverable, no FederatedCallEntry created', async () => {
+    // Arrange: no local DM, no local user matching the participant list
+    const { processRelayEvents } = await importSUT();
+    const cm = await importManager();
+
+    const federatedId = 'fed-call-pathB-empty';
+    const event = {
+      eventType: 'dm_call_start' as const,
+      messageId: 'msg-1',
+      encryptionVersion: 0 as const,
+      timestamp: Date.now(),
+      federatedId,
+      call: {
+        livekitUrl: 'wss://lk.example',
+        tokens: { 'caller-home': 'tok-c', 'unknown-home': 'tok-u' },
+        caller: {
+          homeUserId: 'caller-home',
+          homeInstance: 'https://remote.example',
+          displayName: 'Caller',
+        },
+        participants: [
+          { homeUserId: 'caller-home', homeInstance: 'https://remote.example', displayName: 'Caller' },
+          { homeUserId: 'unknown-home', homeInstance: 'https://remote.example', displayName: 'Unknown' },
+        ],
+      },
+    };
+
+    // Act
+    const result = await processRelayEvents([event], 'https://remote.example', 'https://remote.example', testDb);
+
+    // Assert
+    expect(result.accepted).toEqual([]);
+    expect(result.rejected).toEqual([]);
+    expect(result.undeliverable).toEqual([
+      { messageId: 'msg-1', reason: 'no_recipient' },
+    ]);
+    expect(cm.getFederatedCall(federatedId)).toBeUndefined();
+  });
+});
diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index f2c450c3..499500a6 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -4402,8 +4402,11 @@ function processDmCallStartEvent(
     }
 
     if (ringedUserIds.length === 0) {
-      // No connected users found — silently accept (not an error)
-      accepted.push(event.messageId);
+      // No recipient reachable — signal to caller via third ack bucket (#18).
+      // The remote processed the event cleanly; this is not a data error, but
+      // the caller must learn that nobody was rung so it can tear down its
+      // local ring room instead of hanging 60s waiting for an accept.
+      undeliverable.push({ messageId: event.messageId, reason: 'no_recipient' });
       return;
     }