Cust_ID,Cust_Name and Phone number but am going to write insert command i give only Customer_ID and Cust_Name that time it will show the error invalid

That must because those columns may have been given as non null and you are omitting values for them. In other words, you are not giving values for them, which is not allowed. So the solution would be to make those columns allow nulls. You can do this in the design view of your table or through code.