USER_DEBUG [54]|DEBUG|[{"message":"Session expired or invalid","errorCode":"INVALID_SESSION_ID"}] using Named Credential: 


    Request code -
    
                HttpRequest feedRequest = new HttpRequest();
                feedRequest.setEndpoint('callout:full_dev_connection/services/apexrest/v1/getContacts');
                feedRequest.setMethod('GET');
                Http http = new Http();
                HTTPResponse res1= http.send(feedRequest);
                System.debug(res1.getBody());
                
                
>>>Destination org code for calling
    -
  @RestResource(urlMapping='/v1/getContacts/*')
   global with sharing class getContact {
     @Httpget
      global static list<contact> fetchAccount(){
        RestRequest req = RestContext.request;
        RestResponse res = Restcontext.response;
        Id accId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);
    
        list<contact> lstcontact =[Select id , name,Phone,Fax,Email from contact where Accountid='001O000000Yk7tl'];
        
        return lstcontact ;
      }
   }