BATTERY LIFE: Enabling GPS on your phone typically decreases the battery life by about 50%. This means the phone will need to be recharged twice as often in order to monitor the device for the full day.
TAMPERING: With a GPS-enabled cell phone, the person responsible for carrying the phone is also responsible for ensuring the phone can get a good signal at all times. At times, the java applet running the GPS software on the phone can be turned off, or the cell phone could be placed in a container that blocks the signal.
COST: GPS-enabled phones require a minimum monthly voice plan, a data plan, plus monthly tracking services. These expenses do not exist with a standalone GPS device.