You don't have to have a phone subscription to use a spy microphone. It is quite possible to spy on conversations without a subscription. Indeed, this characteristic is unique to GSM spy microphones. Other types of spy microphones depend on other factors while fulfilling their function.  

The usefulness of a phone subscription for a spy microphone.

GSM spy microphones are microphones equipped with a SIM card. It is this SIM card that will allow thelistening from a distance. In this case, just call the spy microphone for the Microphone open and the spied conversation is audible. It works like any call, so. At this point, no package is required for the operation of the microphone. But in case it is configured to send SMS alerts to the user (to notify them that a conversation has started), a telephone subscription (prepaid sim card or low-priced subscription) will be required. 

How do micro-spies work without a phone subscription?

Aside from the micro spy GSM, no subscription interferes with the smooth running of other spy microphones, i.e. recording a conversation and possibly listening remotely. Indeed, recorder microphones for example. record the conversation in a memory card or on their internal memory all you have to do is connect to a computer. The IP spy microphonesthey are connected to the wifi to relay the recording on a server. And the Bluetooth microphones use this technology to allow a Bluetooth speaker to listen to the ambient conversation from a distance. In conclusion, current technologies make the systematic use of mobile phones and, by extension, telephone subscriptions unnecessary. The micro GSM are recommended for discreet remote listening.