I really like my RF-V16, available on EBay. It is very small and lightweight and it has numerous features, such as: Calling to it and conversing with the person, calling from it to a designated number, calling and listening to the surroundings and of course, GPS tracking.
Three months of service cost me $20, less than $7 a month. It takes a micro SIM card and it is supported via AT&T and T-Mobile. Your phone doesn't have to be with these services.
However, it turned out to be what my Mom needs. She doesn't wander away, but she tends to fall and needs help getting up. This was meant for my P2, but I will buy another.
An editorial comment: If you have pets, spouse, kids, cars, etc., a GPS tracker should be mandatory, with the abductions going on today. For $6 a month, it is a bargain. Don't hold me to it, but I believe the weight is 30 grams and it is very small. Lanyard included, micro USB port for recharging the battery and more features including a SOS button.