On occasions when I require audio recording from a telephone, this device has proved both reliable and affordable.
In my case, when using a 'Olympus' digital recorder, the microphone does take some power from the device. However, the microphone settings on the device work (i.m.h.o) best by selecting 'non-stereo' / 'low' microphone sensitivity and to record at least 'SP' recording quality.
This optimises the incoming and outgoing voice levels and gives better quality recordings. The resulting recordings prove clear but clearly limited by the telephone medium.
So in themselves the microphone is fine, but you may find confusion if you need to record both telephone and normal voices? In m.h.o, i found a method around it being to buy two recorders, with each digital recorder set differently.