Having just had a bad experience with another SIP account/softphone app:
Do you have instructions on how to set up the Freshphone app specifically for Samsung Galaxy A2 Core?
(The problem I had was that when the phone was locked for more than a few minutes, push notifications on the softphone wouldn't work. Push notifications were enabled for the app I had. I checked whether the wifi (FTTH) could be sleeping once the screen locked or after a timeout. I also excluded the Samsung push notifications app from sleeping. Checked battery optimiser and performance manager settings. And so on and so on. For hours)
I specifically bought the Samsung phone so my young kids could dial me via a picture of me in contacts (no ability to read or remember phone numbers yet), at reasonable cost.
If you tell me you have instructions and full settings for the A2 core so the phone will actually ring whenever I phone them and the screen has locked... I will happily sign up.
Most of our users use Samsungs and Push Notification works a treat. I have never played with an A2, but it is very quick and free to setup an account and then the Mobile App is also free. So you are welcome to test and if you are not happy just uninstall
We do also give R5 free airtime for the exact purpose of allowing people to test calls for free as well