In short, I had troubles calling some numbers (13xxxx; Australian local rate) from my Internode-provided VoIP line.
I opened an issue about this on a Sunday, in late afternoon.
Less than half an hour later, they called me directly to run some tests, but I missed that. When I ringed them back, seeing I was calling from my mobile, they offered to call back.
After a couple of tests, they identified the problem, and tracked the cause down to a misconfiguration (some numbers were prefixed to the dialled ones; my fault, I’m afraid) in my router (a Fritz!Box, almost certainly the best piece of home network gear I’ve ever had), and guided me through the process of fixing it (deactivating “Adjust Number Format” features in “Telephony/Own Telephone Numbers”).
Problem fixed in less than an hour, on a Sunday evening. Internode’s customer service is great. This is a very refreshing change as compared to the others usually described here in less than flattering terms.