I got this to use with Phonak Savia Art hearing aids that have a t-loop facility. You wear the transmitter round your neck and the cable that comes out from the bottom of the device plugs into an audio device. There are two lengths of connecting cable available: 20cm and 1 meter. I got the 20cm to use with a tiny radio or my mobile phone. I was disappointed with the quality of sound. Nothing like as good as when I use Bluetooth headphones with, say, my iPhone or just standard headphones or ear-buds connected by cable (not wearing my hearing aids in either case). When I use the t-loop facility in a theatre the sound is much better than I get with this transmitter so I don't think the fault lies with my hearing aids.
The neck-loop is quite cumbersome and is no better than a simple cable in convenience. The only advantage I found was that one could switch (using controls for my hearing aid) quickly between t-loop and surround sound so I could respond promptly to someone speaking to me while I was listening to transmitted sound: not enough of an advantage to make me continue to use this device.
I suppose it might be useful in answering a mobile phone if wearing hearing aids, but you'd have to be wearing the device and have it plugged into your phone.