Here's a break down of the DPS numbering system.
For the most part, the THP are the primary uses of the Regional Communications Centers and their unit numbers are four digit numberical. The first digit in their call number is the region to which they are assigned. If the trooper is in region one, their number is 1XXX, region six, thier number is 6XXX, etc. The second position in the call number is for the sub-region. The sub-regions are refered to as A, B, C, etc, but for radio purposes, region A is represented by 1, region B is represented by 2 or 4, and region C is represented by 3, etc. The third number is the sargeant area that the "troop" is assigned to and can cover a several county area.
As an example, say you hear "2464 Lufkin, 10-27 Texas." This transmission is the trooper calling his regional communications center in Lufkin to run a DL check. The 2464 shows that he is in Region 2-B-6 (6 being the sgt. area) and that he is the #4 trooper in that sgt area. You will notice that troopers always state their unit number, then the station they are calling, and then the ten-code for the radio traffic. The station will always respond with their name and then the troopers radio call number. Once the transmission between the trooper and the station is terminated, the station will always end the transmission with their FCC callsign. This may sound stupid, but the actual reason for it is because of the nature of DPS radios. Troopers can only hear the station they are calling and not other troopers so this is their cue that the radio traffic going on is completed.
Here's an example of a trooper running a driver's license check:
"2466 Lufkin, 10-27 Texas."
"Lufkin 2466?"
"18326381, go ahead."
"Lufkin 2466, Smith, Clear, No 10-29. KKH775."