The Irish host Slovenia at Waterford RSC this Friday (2 September 2016) before travelling to Novi Sad to play Serbia a week on Tuesday (6 September).
Both matches are qualifying fixtures ahead of the UEFA U21 Championships.
Cullen's call up means he will miss City's Checkatrade Trophy group match with Stoke City this Tuesday (30 August) and the club's trip to Millwall on Saturday (3 September) in Sky Bet League One.
The 20 year-old, who is on loan to the Bantams from Premier League side West Ham United, already has five caps to his name for Ireland at Under-21 level.