Calls TO/FROM the School Going Straight to Voicemail on iPhone
Enabling LTE on your iPhone will allow phone calls to and from the school to operate properly.
Click the link to see how to change your settings https://support.apple.com/en-ca/HT203124 .
You may need to update these settings anytime you restart your phone, change Sim cards or Update your phone.
If you have tried this, and it still doesn't work, please contact your cell service provider for more help.