It seems there is some lack of info on the net about placing a SIP call with this device. I would like to use the native Android SIP stack and not another app which would remain running and drain some battery.
First, configure the SIP account. Go to phone app->setting->advanced setting->SIP settings->Add account. By checking receive incoming calls, I can just receive them, albeit with some delay.
Now how to place a SIP call. I found no option in the phone app to select whether to use the mobile network or the SIP account to place the call. I had to create a new contact with the contact app, press Add another field, then put the number in the SIP field. This works and makes use of the SIP account.
I read here you can also enter the phone number follow by ‘@’ and follow by “server of your SIP provider” here is an example :15149999999@sip.voip.com. Maybe this is to select which account to use, but currently I am using only one and it works without the “@” at all.