iPhone - https://apps.apple.com/us/app/sessiontalk-sip-softphone/id362501443
Android - https://play.google.com/store/apps/details?id=co.froute.session_chat&hl=en_GB
Create new account, choose 'GENERIC'
Enter all the details on the next screens (examples below)
Account name -
Auth Username -
Click into 'Domain' and 'Proxy' and fill those in too
Click into 'Advanced Settings'
Click into 'Transport'
Set Transport to TLS
Also click into the 'encrypt audio'
Set this as 'if possible'
Ok that, and make sure account enabled at top