I just found these images from ddg image search “dual sim”.
The first picture, a Samsung graphic is showing dual sim capabilities by indicating sim1 as a house icon in the system area icons and sim2 is a building.

The second picture is showing a square with a “1” in it. This may be your unknown icon.

edit: i just noticed that your icon is next to the clock (left side - App side). So it could be just a regular app also.

Dont forget to rule out dual sim phones; indicating sim 1 is active.