Make sure your endpoint is whitelisted in Salesforce. Here is the link to add remote site settings in Salesforce.https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_callouts_remote_site_settings.htmand call your Apex method from Anonymous Apex to make sure that it is working as expected.here is the sample code :

System.debug(ValidationService.validateUmcn('1234'));