Greetings, ccgene.
I see that you're unable to forward phone calls using your iPhone 5s on Consumer Cellular's network. I'm happy to help.
Call forwarding, call waiting, and other call features on iPhone -- Call forwarding is a carrier feature that can be turned on to use. Tap Settings > Phone and make sure it's on. If it is and you can't forward calls, contact Consumer Cellular to make sure there isn't an issue.
Restart your iPhone, iPad, or iPod touch -- If Consumer Cellular finds no issue with their service, restart your iPhone. Once it's returned to the Home screen, test the issue.
How to back up your iPhone, iPad, and iPod touch -- If restarting your iPhone didn't resolve the issue, please be sure to have a fresh backup to keep your personal information safe. In order to back up your Health data, the backup needs to be encrypted. iCloud does this automatically, but iTunes doesn't. If you'd like to use iTunes to back up your iPhone, refer to this article: About encrypted backups in iTunes
If you can't update or restore your iPhone, iPad, or iPod touch -- Even though the title of the article isn't specific to your issue, the section titled "Put your device in recovery mode and set it up again" will walk you through all the steps to reinstall the iOS software on your device. Test the issue once again.
Take care!