Making calls from mobile is not working cleanly right now. We're working with ONSIM on this. Just be patient and calls connect, but the called party may see several calls before they get one that works, over several seconds.
It looks a lot like a firewalling issue in Tango (which is the underlying mobile network), and they are trying to address this. It means one of two call servers are not working, hence 50% of calls are fine right away and 50% take a few seconds (as it tries the other one).
Profiles on all SIMs have been changed to work around this for now. It still needs resolving, but for now it should not be impacting any customers.