Unable to tunnel through proxy. Proxy returns "HTTP/1.1 503 Service responce sending callout to external system from salesforce

Hi Everyone,

I m having the sending callout to the external system from salesforce and getting responce

Unable to tunnel through proxy. Proxy returns "HTTP/1.1 503 Service but  at the same time i can able send callout from postman to same externam sytem and getting repsoce what is expected.

in both case i m using same details like endpoint url, access token etc.

sample code:

 String AccessToken ='123MIAGCSqGSIb3DQEHAqCAMIACAQExC';

Http http1 = new Http();

        HttpRequest req1 = new HttpRequest();

        req1.setEndpoint('http://172.168.1.249/ConVoxCCS/External/external_dialer.php');     

        req1.setMethod('POST');

        req1.setHeader('Authorization','Bearer '+ AccessToken);

        //req1.setHeader('Content-Type','application/x-www-form-urlencoded');

        req1.setHeader('Content-Type','application/json');

String reqbdy = '{"action":"DATAUPLOAD","data":[{"mobile":"8977332322","info_1":"info1","info_2":"info2","info_3":"info3","info_4":"info4","info_5":"info5","info_6":"info7","info_8":"info8","info_9":"info9","info_10":"info10","info_11":"2019-08-29","info_12":"info12","info_13":"info13","info_14":"info14","info_15":"info15","dial_prefix":"102","priority":"10"}, {"mobile":"8977332322","info_1":"info1","info_2":"info2","info_3":"info3","info_4":"info4","info_5":"info5","info_6":"info7","info_8":"info8","info_9":"info9","info_10":"info10","info_11":"info11", "info_12":"info12","info_13":"info13","info_14":"info14","info_15":"info15"}],"refno":"123456789","process":"ConVox_Process"}';

        req1.setBody(reqbdy);

        req1.setTimeout(60000);

        HttpResponse res1 = http1.send(req1);

        System.debug('convx Res1:' + res1);

        System.debug('==>convx Response Code==>'+res1.getStatusCode());

        System.debug('==>convx Response body==>'+res1.getBody());

Any help on this will be much appriciated.