On the SIP accounts page you'll see any SIP accounts you've already created and you can click on them to modify them or click on the '+' to add a new account.
The SIP account details screen has the following options (touch 'Optional settings' to see all available settings):
Username - Enter your sipXcom username (extension typically) here
Password - Enter your SIP password here (not your user password, not your PIN)
Server - Enter the FQDN of your server here (SRV records do not work, if you have a HA setup, you'll need to pick a server or have a SBC setup to connect through. An issue has been opened against it Android Issue ID 184088.)
Authentication username - Leave blank
Display name - How you want other system users to see your name when you call.
Outbound proxy address - Leave blank
Port number - 5060
Transport type - TCP or UDP (TCP is preferred now)
Send keep-alive - Set to 'Always send'
Once you make settings changes always click on the 'SAVE' button at the top of the page!
To setup SIP extensions on your contacts, go into the Contacts app, edit your contact and select 'More fields' at the bottom of the page, this will then reveal the SIP calling address. Enter the user's SIP URI (extension@FQDN remember, it's not smart enough for SRV records)