There may be native support for a CardDAV/CalDAV provider / phonebook available for your device.
If not or you prefer to use a good open source solution we like to point you to:
When you followed the above steps, you can immediately activate the phonebooks you want to sync by toggling the switch, either for 37c3 or EPVPN or both.
To synchronize, click the orange button in the bottom right corner of the screen.
additional note: using android phonebook in baresip(+)
go to settings
set the option 'Contacts' to 'Android' or 'Both' to have the app use the android phonebook.