Phone recording; quality vs. features

Hello,
In looking at different hardware to capture audio from a phone conversation, my main question remains. How does the actual quality of the audio vary from source to source?

I understand all methods, including thousands of dollars of equipment from radio stations still leaves you with the initial bad phone quality, so I'm trying to understand the benefits from a quality-standpoint of using something like a Telos Delta instead of connecting any portable recording device to your phone via the appropriate cables. I know the more advanced equipment has the null option, improves the audio with some EQ, separates lines, etc. but from a pure quality standpoint, what of this could not be gained with doing the same EQ adjustment yourself if you've got a clean feed?

In short, my question is it really a noticable difference? And if so, is it from the EQ and filters that the hardware provides, or is there something I'm missing that a more modest configuration simply can not replicate even with basic post production?