The JAMB (Joint Admissions and Matriculation Board) office in Lagos is one of the institute's two 'headquarter annexes'. The address is as follows: 11 Ojora Road, Ikoyi, P.M.B. 12748, Lagos, Nigeria. The telephone number is 01-2694751/2, the fax number is 234-1-2694098.
The other headquarter annex is in the town of Kaduna. The address: 79 Tafawa Balewa Way (Near Rimi College) P.M.B. 2200, Kaduna, Nigeria. The telephone number is 062-240405/241497/ 241655. The fax number is 234-62-240405.
If you need to contact the national headquarters of JAMB, the details are as follows: Suleja Road, Bwari, P.M.B. 189, Garki, Abuja, Nigeria. Telephone number: 09-8501756/7. Fax number: 234-9-8501284, 5238803.
Here's a recent news report involving JAMB. The organization had been accused in the past of making it easy for cheats to gain entry into Nigerian universities, but JAMB says it has stamped out this practice: