How to factory reset a iphone 6

See these support articles:

Restore your iPhone, iPad, or iPod to factory settings - "A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS or iPod software." - http://support.apple.com/HT201252

How to erase your iPhone, iPad, or iPod touch

"Learn how to wipe information from your iPhone, iPad, or iPod touch and restore your device to factory settings." Without using a computer. Does not reinstall iOS.

https://support.apple.com/HT201274

May require a possible additional passcode if the SIM card uses one.

https://communities.apple.com/de/thread/250297605?answerId=250558115622#250558115622

Contact your IT staff for help with this. Since it is a work phone, it may have a mobile device management profile that would need to be deleted so that you can use it with the new business.