Unfortunately some restrictions seem necessary: You may only search for TACs, requests will be lagged depending on access frequency. Not more than 4 requests per minute, not more than 40 requests per 24 hours. No access with scripts or programs, no commercial usage. Anyone in need of unlimited access may use the PRO-Database.
TAC: the TAC consists, depending on the age of the device, of the first 6 or 8 numbers of the IMEI.
IMEI: the mapping IMEI - Terminal and the IMEI validity check is here.