Telephone icon

If there is a phone icon with a an arrow pointing right, it means call forwarding is on.

If there is a phone icon resting on some squares, it means Teletype is on.

See here: Status icons and symbols on your iPhone - Apple Support

A telephone icon where?

The phone App has a telephone icon which means its used to make apps:

There could be a telephone icon in the contacts App, you can tap on to call that specific contact.

Where are you seeing this phone icon you are concerned about exactly?

The Phone app is a white telephone icon on a green background. I would think you know that already unless you are brand new to an iPhone.

If you are talking about something else, where are you seeing this icon?

Does it look like this? Call forwarding?

https://help.apple.com/iphone/11/#/ipha441f15a