Lammert has more than 35 years of telecom and tax experience, which started in the competitive environment following the 1984 breakup of AT&T and the Bell telephone system. Lammert is a CPA in Florida (1991) and Missouri (1987). He is a member of the American Institute of Certified Public Accountants.

Lammert joined the Tulsa office of Arthur Young & Company in May 1987 (Ernst & Young 1989). He spent more than six years in the 1990s as a Tax Manager at an Incumbent Local Exchange Carrier. During his career, he has worked in various capacities in public and private accounting ventures, including internal audit, accounting management roles, strategic business planning, and the creation of systems and processes for tax compliance. While in public accounting, Lammert served many audit clients in various industries, such as telecommunications, retail, construction accounting, and public utilities. He has significant experience with all-sized clients and different support roles for the clients.