Siddiqi undertook post doctoral studies at Heidelberg University under the guidance of Horst Leptin (1971–72) as a fellow of German Academic Exchange Service.

He had held visiting assignments in different parts of the world including USA, Canada, Germany, Iran, Algeria, Saudi Arabia, Oman and Malaysia.

[8] Siddiqi was awarded a German Academic Exchange Service (DAAD) Fellowship in 1971 for postdoctoral work at Institute of Applied Mathematics at Heidelberg University for a period of eighteen months.

He was also awarded DAAD revisit fellowships during 1989 and 1997 to work at the Institute of Industrial and Business Mathematics, University of Kaiserslautern, for period of three months, each time.

Siddiqi was also awarded regular Associateship of the International Centre for Theoretical Physics (ICTP) in Trieste, Italy, on the basis of worldwide competition for the period of 1992 to 1997.