ONSIM are making changes to address some long standing issues with slow connects and multiple connects for outgoing calls. Sorry for the short notice, they have just advised us. We have been nagging them to fix this for some time. It is possible some calls from mobiles may fail, but it is worth retrying if they do. We are hoping this will only last a few minutes, but not depends on the mobile carrier completing the work promptly.
We have not had feedback, but tests suggest calls are being consistently reliable now.