Install the following app.

SessionTALK

and use your extension details 
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'

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Enter all the details on the next screens (examples below)

Account name - 256
Username - 
Password - 
Display - 
Auth Username -

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Click into 'Domain' and 'Proxy' and fill those in too

Domain - jjpbx.co.uk
Proxy - jjpbx.co.uk

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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

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