VoIP virtual rooms, mk II

Does a thirdparty protocol lookup to the homeserver to get the
corresponding native/virtual user for a matrix ID. Stores the
mappings in room account data. Involves some slightly nasty workarounds
for that fact that room account data has no local echo.

src/SlashCommands.tsx

@@ -1040,9 +1040,7 @@ export const Commands = [
 
             return success((async () => {
                 if (isPhoneNumber) {
-                    const results = await MatrixClientPeg.get().getThirdpartyUser('im.vector.protocol.pstn', {
-                        'm.id.phone': userId,
-                    });
+                    const results = await CallHandler.sharedInstance().pstnLookup(this.state.value);
                     if (!results || results.length === 0 || !results[0].userid) {
                         throw new Error("Unable to find Matrix ID for phone number");
                     }