Heterodyne detectors are by far the most affordable & easy-to-use. They are designed to be tuned to a frequency range of your choice. Calls are then converted into audible sounds that you can listen to! They are perfect for beginners.
If you are looking for something that can record sounds for post-visit analysis why not take a look at the Frequency Division range. Some models have dual modes & also have Heterodyne functionality.
Brands on offer include Pettersson, Batbox, Elekon & Magento.