Apple - What is this little "pay phone" icon in the status bar?

Its the TTY icon, according to this Apple KB. You can go to Settings -> Phone to turn it off.

TTY, Teletype (or various other names for it) is a special device you can use on a phone to help those that are deaf or hard of hearing communicate. Not sure why it would be enabled with the new SIM, unless there was something on the account on the new SIM that enabled this, and Apple did something to read this setting and configure their own setting.

You can safely turn it off if you are not using one of these machines.

From the iPhone User Guide

" Shows that iPhone is set to work with a TTY machine."

Connect iPhone to a TTY machine:  Go to Settings > Phone and turn TTY on, 
and then connect iPhone to your TTY machine using the iPhone TTY Adapter.

When TTY is enabled on iPhone, the TTY icon ( ) appears in the status bar at the top
of the screen.

This is from the User guide. I can't find that particular setting on my iPhone...

The icon looks like that because of what some of the TTY devices looked like: