I always try to switch it off out of respect for my Domme & not wanting to disturb the flow. However, I also don't have any specific reason to leave it on as you do.

Only downside I see either way (since you aren't answering it even if your partner calls), is forgetting to turn it back on after the session. Not sure how your partner will know if you have it turned off versus putting it on "Do not Disturb" or just ignoring it because you were "busy" during that time but everyone's situation is different.
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Asp