When you do an INSERT query, it's a very good idea to always list the columns you are going to insert the data into, or SQL will just start with the current first column and insert your values starting with that.
In your case, that first column is an IDENTITY column - so even if you did supply an integer value, it still would throw an exception.
Try:
INSERT INTO mst_reg (Name, Lastname, Fathername, Qualifiaction, Emailid, Phno) VALUES ('aqib', 'javid', 'nijamudeen', 'BTechIT', 'aqib55@live.com', 965557017796)
But please, if you are doing this via a high level language, remember to use parameterized queries! Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead