Two Sequences on one column
Create two sequences, one for IBM and one for Airtel. Set the start points appropriately. Put all the logic in a stored procedure. Pick the right one for each INSERT.

This is not a great design. Surrogate keys like this are best if they are completely meaningless. By tying a range of values to a customer you may have problems in the future. What will you do when a range is exhausted? What if you acquire another customer? Will you tolerate gaps in the sequence? For a short-lived, single-purpose application you may be OK. For an enterprise application it is a risk.