diff --git a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
index d0ca37b2d85..2b391661433 100644
--- a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
+++ b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
@@ -399,6 +399,42 @@ describe("MatrixRTCSession", () => {
             jest.useRealTimers();
         });
 
+        describe("non-legacy calls", () => {
+            const activeFocusConfig = { type: "livekit", livekit_service_url: "https://active.url" };
+            const activeFocus = { type: "livekit", focus_selection: "oldest_membership" };
+
+            function testJoin(useOwnedStateEvents: boolean): void {
+                if (useOwnedStateEvents) {
+                    mockRoom.getVersion = jest.fn().mockReturnValue("org.matrix.msc3779.default");
+                }
+
+                jest.useFakeTimers();
+                sess!.joinRoomSession([activeFocusConfig], activeFocus, { useLegacyMemberEvents: false });
+                expect(client.sendStateEvent).toHaveBeenCalledWith(
+                    mockRoom!.roomId,
+                    EventType.GroupCallMemberPrefix,
+                    {
+                        application: "m.call",
+                        scope: "m.room",
+                        call_id: "",
+                        device_id: "AAAAAAA",
+                        foci_preferred: [activeFocusConfig],
+                        focus_active: activeFocus,
+                    } satisfies SessionMembershipData,
+                    `${!useOwnedStateEvents ? "_" : ""}@alice:example.org_AAAAAAA`,
+                );
+                jest.useRealTimers();
+            }
+
+            it("sends a membership event with session payload when joining a non-legacy call", () => {
+                testJoin(false);
+            });
+
+            it("does not prefix the state key with _ for rooms that support user-owned state events", () => {
+                testJoin(true);
+            });
+        });
+
         it("does nothing if join called when already joined", () => {
             sess!.joinRoomSession([mockFocus], mockFocus);
 
diff --git a/spec/unit/matrixrtc/mocks.ts b/spec/unit/matrixrtc/mocks.ts
index 7d63b32b79a..c11544a0e9e 100644
--- a/spec/unit/matrixrtc/mocks.ts
+++ b/spec/unit/matrixrtc/mocks.ts
@@ -30,6 +30,7 @@ export function makeMockRoom(membershipData: MembershipData, localAge: number |
         getLiveTimeline: jest.fn().mockReturnValue({
             getState: jest.fn().mockReturnValue(roomState),
         }),
+        getVersion: jest.fn().mockReturnValue("default"),
     } as unknown as Room;
 }
 
diff --git a/src/matrixrtc/MatrixRTCSession.ts b/src/matrixrtc/MatrixRTCSession.ts
index 2f06eba633a..cc155fd3e6a 100644
--- a/src/matrixrtc/MatrixRTCSession.ts
+++ b/src/matrixrtc/MatrixRTCSession.ts
@@ -869,7 +869,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
                 this.room.roomId,
                 EventType.GroupCallMemberPrefix,
                 newContent,
-                legacy ? localUserId : `${localUserId}_${localDeviceId}`,
+                legacy ? localUserId : this.makeMembershipStateKey(localUserId, localDeviceId),
             );
             logger.info(`Sent updated call member event.`);
 
@@ -899,6 +899,15 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         return false;
     }
 
+    private makeMembershipStateKey(localUserId: string, localDeviceId: string): string {
+        const stateKey = `${localUserId}_${localDeviceId}`;
+        if (/^org\.matrix\.msc3779\b/.exec(this.room.getVersion())) {
+            return stateKey;
+        } else {
+            return `_${stateKey}`;
+        }
+    }
+
     private onRotateKeyTimeout = (): void => {
         if (!this.manageMediaKeys) return;