I built one using these: 1) Single Door Magnetic Locks Electromagnetic Lock 600LB 12V Door Lock, about $30.00 with shipping, 2) GSM Controller remote control Switch Relay, about $20.00 3) 12 volt battery with a solar charger, 4) Cell Phone for you and one for the GSM Controller - It only need to receive text messages. A cell phone can receive text messages much easier than a call so even the most sketchy place works.
Before I want to hunt I place the cell phone in a plastic box (keeping it weather proof), with the GSM Controller which is connected to the magnetic lock, 12 volt battery and solar charger that holds up my 4' wide gravity drop cattle panel gate. I set up within view of the hog trap that also has a feeder in it. When they arrive, and usually right on time for the feeder to go off, I send a text message to the other cell phone, within a second or two it receives the text message, the GSM Controller then turns off the connection to the Magnetic lock and the cattle panel falls trapping them for me to take of. It has been up and running for about a year, I've had no problems or failures with it so far and if needed replacement parts are cheap and easy to replace.