The GSM systems by Estate Swing, EST-1000 intercom and the EST-64 receiver, require a GSM compatible SIM card. You can either add a line to your existing plan or go with a pay as you go phone. The key is to get a SIM card that is compatible with GSM technology. The two biggest carriers to offer such a card are AT&T and T-Mobile. The SIM card must be able to send and receive text messages, have minutes available, be unlocked, and have a working phone number. The GSM intercom and receiver unit are basically working cell phones that receive calls and text messages. The GSM interom also makes calls to up to three phone numbers to allow access to the gate remotley. For instance, a guest pulls up to the gate and he/she hits the call button. The GSM then rings the administrator phone number and up to two other manager numbers if the admin. does not answer the phone. The admin. or the manager can then grant access to the gate no matter where he/she is, as long as they have cell phone coverage.