Horizon App (Old) - Phone doesn't ring on inbound Calls / Can't make outbound Calls

Horizon App not ringing? What is it doing?

Ringing briefly, then dropping. The call registers in your missed calls list.
Not Ringing at all. The call registers in your missed calls list.

Check the following:

1. Are you on Wi-fi? We'd normally suggest you would have your phone on Wi-fi for data usage reasons, but this will cause issues with the app. When on Wi-fi, your Horizon app is unable to communicate properly with the cloud server as your home router imposes traffic restrictions for unsolicited inbound traffic.

2. Have you checked your presence setting? Tap the 3 Line menu icon, and check the status beneath your name (should read "Mobile") unless you are in a call. Press the down arrow beside this and choose "Automatic"

3. Is your phone on DND or call forwarding mode? Tap the 3 Line menu icon and look near the bottom of the menu for "Settings". Tap here. Check the following:

Do not Disturb is toggled off
Tap Call Forwarding and ensure that "Always Forward" is off
Calling should be set to VOIP.

Try the following:

Restart your phone if it hasn't been restarted already. Try for a while and see how it behaves.
On your phone, swipe down and tap the gear to enter Settings. Proceed to Apps > Horizon > Choose the option marked "Force Stop" (if available) to force quit the app.
Still in the Horizon app page, Choose "Storage" and choose the option marked "Clear Data"
Go back to Horizon and re-enter your username and password. Your username is ext###@queenscrosshousing.co.uk (where ### is your 3 digit extension number) and copy/paste your password from the latest email you have from noreply@unlimitedhorizon.co.uk (you can request a password reset from ICT, the reset password feature from the app will NOT work)
Re-accept all permission prompts (Choose Allow).
Re-enter your mobile phone number (this is provided for fallback reasons should the VOIP platform fail for any reason)