That's different from a "serious tone of voice" though. Think like the way a parent might start talking to their kid when they're angry but not yelling, with something like "You better get home right now."

Or another example: "Call me" is a just a regular "let's chat about something", but "Call me." is "something bad happened I need to tell you about, so prepare yourself".

Interestingly, you're actually partially doing what I described on 2 of your 3 messages in this chain - you left out the last period because HN formatting makes it obvious where the sentence ends. So even if this norm did apply here (it doesn't really), you're not using the serious tone of voice.

This is interesting. I didn't realise so many people disable it. A lot of what you're saying is completely backwards to my life experience.

For me and I guess most people I communicate with on e.g. Whatsapp. "Call me." is normal, expected, everything is fine, just need a phone call. "call me" is more like something has gone so horribly wrong (or someone is so incredibly pissed off) they've lost the ability to communicate normally. I wouldn't be offended, more like concerned.