diff --git a/src/stores/widgets/StopGapWidgetDriver.ts b/src/stores/widgets/StopGapWidgetDriver.ts
index d47c9d18e42..aa8721c28e8 100644
--- a/src/stores/widgets/StopGapWidgetDriver.ts
+++ b/src/stores/widgets/StopGapWidgetDriver.ts
@@ -144,6 +144,7 @@ export class StopGapWidgetDriver extends WidgetDriver {
             const clientDeviceId = MatrixClientPeg.safeGet().getDeviceId();
             if (clientDeviceId !== null) {
                 // For the session membership type compliant with MSC4143
+                // Note MSC4143 still uses the org.matrix.msc3401 unstable prefix
                 this.allowedCapabilities.add(
                     WidgetEventCapability.forStateEvent(
                         EventDirection.Send,
@@ -151,6 +152,13 @@ export class StopGapWidgetDriver extends WidgetDriver {
                         `_${clientUserId}_${clientDeviceId}`,
                     ).raw,
                 );
+                this.allowedCapabilities.add(
+                    WidgetEventCapability.forStateEvent(
+                        EventDirection.Send,
+                        "org.matrix.msc3401.call.member",
+                        `_${clientUserId}_${clientDeviceId}_m.call`,
+                    ).raw,
+                );
                 // Version with no leading underscore, for room versions whose auth rules allow it
                 this.allowedCapabilities.add(
                     WidgetEventCapability.forStateEvent(
@@ -159,6 +167,13 @@ export class StopGapWidgetDriver extends WidgetDriver {
                         `${clientUserId}_${clientDeviceId}`,
                     ).raw,
                 );
+                this.allowedCapabilities.add(
+                    WidgetEventCapability.forStateEvent(
+                        EventDirection.Send,
+                        "org.matrix.msc3401.call.member",
+                        `${clientUserId}_${clientDeviceId}_m.call`,
+                    ).raw,
+                );
             }
             this.allowedCapabilities.add(
                 WidgetEventCapability.forStateEvent(EventDirection.Receive, "org.matrix.msc3401.call.member").raw,
diff --git a/test/unit-tests/stores/widgets/StopGapWidgetDriver-test.ts b/test/unit-tests/stores/widgets/StopGapWidgetDriver-test.ts
index ec352f28d04..e654cdfea54 100644
--- a/test/unit-tests/stores/widgets/StopGapWidgetDriver-test.ts
+++ b/test/unit-tests/stores/widgets/StopGapWidgetDriver-test.ts
@@ -110,6 +110,8 @@ describe("StopGapWidgetDriver", () => {
             "org.matrix.msc2762.receive.state_event:org.matrix.msc3401.call.member",
             `org.matrix.msc2762.send.state_event:org.matrix.msc3401.call.member#_@alice:example.org_${client.deviceId}`,
             `org.matrix.msc2762.send.state_event:org.matrix.msc3401.call.member#@alice:example.org_${client.deviceId}`,
+            `org.matrix.msc2762.send.state_event:org.matrix.msc3401.call.member#_@alice:example.org_${client.deviceId}_m.call`,
+            `org.matrix.msc2762.send.state_event:org.matrix.msc3401.call.member#@alice:example.org_${client.deviceId}_m.call`,
             "org.matrix.msc3819.send.to_device:m.call.invite",
             "org.matrix.msc3819.receive.to_device:m.call.invite",
             "org.matrix.msc3819.send.to_device:m.call.candidates",