I'll note that since MoP Trash in raids has become more mechanic oriented than ever before and it seems to just ratchet up from there. In Emerald Dream you have circles the group has to get into after the first boss or people die on the tree bosses, and others that people have to get out of. The slimes require people to step away from the group and walk away from big bugs. In MoP there were ghosts on the walkways that stacked -heal % on the entire group and if they were not killed fast enough could wipe the group.
So yeah I am not surprised a group would want you to not play with your phone during trash. If more people did it it would make trash a nightmare for the rest of us.
I will note 2 things.
1. Everyone justifies the shitty things they do. They make up reasons in their heads as to why it is okay for them to do something. A Bank Robber thinks, "I need the money, it is ensured, victimless crime." but the bank teller who gets the gun waved in their face and thinks they are going to die doesn't think it is a victimless crime.
2. We only get the story from OP's point of view, thus we don't really know what happened. The story will be twisted just slightly to paint themselves in a more positive light as they are incapable of experiencing life from another's perspective. "I checked my phone from time to time."= "I spend the rest of trash texting my friends because I don't care about trash."
Your guild came to you and said 'fix this or you are out', obviously it is an issue and has happened a lot. Your options are to fix it or be removed. Figure out what you want to do.
Like I said, Trash is no longer spam AOE. If you are getting killed by trash mechanics because you are not paying attention then yeah, your guild having to carry you and wait for you is costing them their enjoyment.