how to manage my at commands

hi i have this codes

With SerialPort1
                Try
                   
                    .Write("at" & vbCrLf)
                    Threading.Thread.Sleep(1000)
                    
                    .Write("AT+CMGF=1" & vbCrLf)
                    Threading.Thread.Sleep(1000)
                    
                    .Write("AT+CPMS=""SM""" & vbCrLf)
                    Threading.Thread.Sleep(1000)
                    
                    .ReadTimeout = 1000
                    .Write("AT+CMGL=""all""" & vbCrLf)
                    Threading.Thread.Sleep(1000)

i read at command output by this

SerialPort1.ReadExisting

heres the error. i can read messages but the problem is it doesnt read all message

but when set the interval of Threading.Thread.Sleep(10000) it reads all the messages

how can i manage the at commands so that it will stop reading messages when all are done?