Re: Not office hours? No contact
It really is that easy for a lot of people — don't add your work accounts to your phone. Don't do it because you feel you have to prove something, don't do it because the person that sits next to you does it, and encourage others not to do so as well. If you have a work mobile then maybe there's an out-of-hours element to your job, but it should be clearly stated in your contract.
If nothing else, doing free bits of work on a regular basis is dishonest to your employer. How are they ever going to know (or care) that they're under-resourced if people keep picking up the slack, or indeed the Slack?