"caller":"caller:123"
"callee":"callee:123"

mutate {
          gsub => [
                   #regex to remove : and every thing before it
                   "caller", "^[^:]*:", "",
                   "callee","^[^:]*:", ""
                  ]
          convert => {
                     #type conversion from string to integer
                       "caller" => "integer"
                       "callee" => "integer"
                     }
        }

mutate {
              gsub => [
                       #regex to remove : and every thing before it
                       "caller", "^[^:]*:", "",
                       "callee","^[^:]*:", ""
                      ]
       }

           mutate {
                     convert => { "caller" => "integer"
                                  "callee"=>  "integer"
                                 }

                   }

def filter(event)
    ...
    convert(event) if @convert
    gsub(event) if @gsub