espocrm-base/application/Modules/Crm/Metadata/viewDefs/Call.json

{
   "sidePanels":{
      "detail":[
         {
            "name":"attendees",
            "label":"Attendees",
            "view":"Record.Panels.Side",
            "options":{
               "fields":[
                  "users",
                  "contacts",
                  "leads"
               ],
               "mode":"detail"
            }
         }
      ],
      "edit":[
         {
            "name":"attendees",
            "label":"Attendees",
            "view":"Record.Panels.Side",
            "options":{
               "fields":[
                  "users",
                  "contacts",
                  "leads"
               ],
               "mode":"edit"
            }
         }
      ]
   },
   "menu":{
      "list":{
         "buttons":[
            {
               "label":"Create Call",
               "link":"#call/create",
               "style":"primary",
               "acl":"edit"
            }
         ]
      }
   }
}