It has been confirmed that at some point SIP functionality will be integrated in the Jolla.

https://together.jolla.com/question/...e-integration/

For the time being you could use a SIP client for Android to call other SIP accounts.

I have tested CSIPSimple , the nicest SIP client on Android but it doesn't work due to the high level of integration into Android (which makes it so good)

2nd I have tested Jitsi for Android client.
Jitsi is still in alpha but it seems to work and can call other SIP accounts.
It has great configuration options and I like particularly the DNS option that allows to sneak past the Google DNS that is hard coded in the Android part of the Jolla.
I hope Jitsi will soon turn to native Sailfish.

Note that for now the Android programs such as Skype or Jitsi use the bottom loudspeaker for sound output and not the phone's earspeaker.

Let's dump MS Skype (and all other proprietary services such as Viber etc..) in favour of SIP and XMPP. Together we can do it.