Cell phone audio quality sounds different on analog VS VoIP

Can someone help me understand what causes a poor cell phone service to sound choppy on VoIP where it would be noisy on analog?

Not much to go on here. Can you explain the scenario for each type of call in more detail and then maybe someone can help.

I often speak to different tech support people about call quality and when I say choppy and cell phone in the same sentence, they quit and say, well that is just how cell phones and VoIP work sometimes.
My customers understand that cell phones sometimes sound terrible even on analog. The problem is, when I switch someone from analog to VoIP bad cell phones sound different.
I am guessing that there are background noise filters build into the VoIP process and that results in errors during really noisy calls.
My hope is that someone smarter than me can confirm this or correct me.

Thanks for reading my convoluted question!