Callert served in
Norrland Artillery Regiment for about ten years. He was promoted to captain in 1992 and major in 1996. During this time, Callert attended the General Course at
Military Academy Karlberg in Stockholm from 1989 to 1990, the Higher Course at the
Artillery and Engineering College in Stockholm from 1992 to 1992 and the General Course, tactical course for the Army at the
Swedish Armed Forces Staff College in Stockholm from 1995 to 1996. Thereafter, Callert served from 2008 to 2010 as head of the Analysis and Follow-up Section in the Joint Forces Command at the Swedish Armed Forces Headquarters in
Stockholm. In 2010, he was promoted to colonel and served as regimental commander of the Artillery Regiment from 2010 to 2014. From 1 February 2014, he served in the Implementation Department (
Genomförandeavdelningen) of
Training & Procurement Staff at the Headquarters, for a period in 2014 as deputy head and from 2014 to 2016 as head. Callert was promoted to brigadier general in 2016 and from 1 June 2016, he served as
Assistant Chief of Armed Forces Training & Procurement in the Training & Procurement Staff at the Headquarters. He was promoted to major general in 2017. Callert assumed the position of head of the Swedish delegation to the
Neutral Nations Supervisory Commission in
Korea on 1 June 2021. On 23 October 2023, Callert assumed the position of Deputy Vice-Chancellor of the
Swedish Defence University. ==Dates of rank==