Here are some suggestions for cable clamp ferrites. You can get them mail order from DigiKey, they are cheap really. Note the lossy impedance ratings, so the one that is slightly more expensive is somewhat better if you buy just one. That rating is at the frequency specified, at audio frequencies the ferrites are non existent. The ones with larger holes you can wrap the cable around and go through it twice, but for phone cables, I'd just buy a few extra as I mentioned above.
https://www.digikey.com/en/products/detail/fair-rite-products-corp/0443164251/8594062
https://www.digikey.com/en/products/detail/w%C3%BCrth-elektronik/7427141/9858467
BTW, the reason you get more pickup when you hold the cable next to the amp is your body is acting like an antenna, whereas at the opposite end, it acts like a load. There is a reason why RF is often referred to as Black Magic! Once you sort out the problem and find the solution, you will discover it is pure science.