How do I know if my iPhone SIM card has been removed?

I lost my iPhone at a crowed event a few days ago and I'm unable to track it on Find My iPhone, because it is offline. However when I call my phone number it still goes straight to my voicemail. Has my iPhone been stolen with the SIM card removed or is the SIM card still there?

When the call goes to your voicemail, that usually mean that the phone can't or won't accept the call. This may be because this number (or all numbers) have been set to go directly to voicemail or the network can't reach the phone. Reasons for the latter include the phone being off, the phone simply being out of network range or the SIM being removed from the phone.

You can't typically remove the SIM from an iPhone.
and you can leave a message even when the phone is off. And find my phone doesn't work when the phone is off, if its battery is drained, or if itxs in an area blocked from GPS signal.