Wifi Calling on Galaxy S9 Plus not automatically swiching on

Hi, Does anyone know if there is a way to force wifi calling on a Galaxy S 9 plus? I currently have to restart the phone for this option while indoors at work where there is no mobile signal and when it connects it works very well. If I leave the wifi network and go outside where there is a weak but usable network signal the phone automatically switches to the mobile network but won`t switch back to wifi calling when it reconnects to the wifi network. Switching wifi calling off and on again doesn't work. The issue is also the same on my home wifi too although I do have a mobile signal there. To make this more anoying my lesser spec Samsung work phone on the Vodaphone network switches seamlessly?