The accused, who is above 50 years of age and is an athletics coach, is currently lodged in Tihar Jail under the POCSO Act.
Further investigation and forensic analysis will be done after the mobile handset and details of the seizure are received from Tihar Jail authorities.
Akshay Thakur, 31, Pawan Gupta, 25, Vinay Sharma, 26, and Mukesh Singh, 32, were executed by Pawan Jallad, the hangman from Uttar Pradesh's Meerut district.
The black warrants, which were issued for the first time on January 7, were deferred four times earlier on the ground that they were yet to exhaust all the legal remedies.
As part of the ritual before hanging, the jail authorities practice a dummy execution with sandbags according to the prisoner's weight.
On Tuesday, a Delhi court issued death warrants against Mukesh (32), Pawan Gupta (25), Vinay Sharma (26) and Akshay Kumar Singh (31) and said they will be hanged on January 22 at 7 am in Tihar jail.
According to a court order issued on Tuesday, the four convicts are to be hanged on January 22 at 7am.
Nirbhaya, a 23-year-old paramedic student, was gangraped and brutalised on the intervening night of December 16-17, 2012, inside a moving bus in south Delhi by the four men, including two others, before being dumped on the road.
Shakeel, who operates along with his boss Dawood Ibrahim from his hideout in Karachi, has allegedly tasked an Indian module of the D-company to eliminate Rajan inside his Tihar cell.
Director General (DG) of UP Prisons, Anand Kumar has confirmed that the Tihar jail superintendent had written a letter to the Uttar Pradesh prison authorities seeking services of two hangmen.