[2] From August 1996 to April 1997, Callert served as Deputy Section Chief of S3, in the Swedish battalion (SWEBAT, BA07) headquarters, part of the Stabilisation Force in Bosnia and Herzegovina (SFOR), a NATO-led multinational peacekeeping force deployed to Bosnia and Herzegovina after the Bosnian War.

[2] Callert attended the Advanced Command Program at the Swedish Armed Forces Staff College from 1998 to 2000, with profiling in protection and countermeasures.

He then served as Army Tactical Command as a staff officer from June 2001 to December 2002, responsible för indirect fire.

[2] From January 2003 to March 2004, Callert served as a staff Officer responsible for indirect fire at Swedish Armed Forces Headquarters in Stockholm.

[9] Callert assumed the position of head of the Swedish delegation to the Neutral Nations Supervisory Commission in Korea on 1 June 2021.