I agree with you. He was holding on to the cell phone with something more than just sentimental value. By not getting rid of the phone and even yelling at Kanchan to not touch it, despite it not even working, gave me the feeling that Yumi wasn't letting go of the past.
I have a sinking feeling that even if the abuse and stress hadn't taken place, Yumi would have still held on to the memory of Mayama and eventually got with him.
I'm glad Yumi and Kanchan broke up (it's unfortunate, cause I think the two were good together before all the abuse and hurt started), but the case with the cell phone and Yumi never having gotten over him while being in a long term relationship (before the abuse) doesn't sit with me well.
I’m still really bothered by how Yumi basically kept the cellphone that he promised to call Mayama with even though he’s together with Kanchan for so long. He doesn’t even let kanchan touch it. Idk maybe I’m thinking too much but I always have seen Yumi taking kanchan as a substitute for mayama because he was lonely/swept away but has always been hold feelings for mayama all these time. If kanchan never abused yumi and the author went on with the Sabita series, it will definitely make it seem like Yumi is a bad person. No matter how you see it, Yumi always harboured feelings for Mayama and could never let go while kanchan is loving him and trying to support him with all his heart.