In the Bria app go to accounts and tap the "+" button.
Select SIP from the "User-defined Generic Accounts" heading.
This will bring up a screen that says "New SIP Account".

Account Name: WhiteSky PBX

User Details
Display As: <YOUR NAME>
Username: <YOUR EXTENSION>
Password: <EXTENSION PASSWORD>
Domain: <PBX DOMAIN NAME e.g. pbxXXXX.voice.wtsky.net>

Voice Mail
VM Number: *97

Navigate to Account Advanced below "Voice Mail"

Advanced Settings
Account Additional
**Leave these at their defaults

DTMF Type
Send DTMF using: RFC 2833

Media Network Traversal
Suppress STUN Wi-Fi: Off
Suppress STUN Mobile: Off
Use ICE Wi-Fi: Off
Use ICE Mobile: Off

SIP Network Traversal
Global IP WI-FI: Off
Global IP Mobile: Off

Call Dialing
Use Tel Uri: Off

Transport and Security
SIP Transport: TCP
Encrypt Audio: Never

SIP Registration
Incoming Calls: On
Refresh Interval: 600

Keep Alive
WI-FI Interval: 60
Mobile Interval: 15

SIP Miscellaneous
Show Miscellaneous: On
Single Register: Off
Passive Session Timer: On
Connection Reuse: On
Enable IMS: Off