Well the first thing I would say i that there is no way of just upgrading the upstream speed; as you have found this can only be done by upgrading your entire subscription.
Now unfortunately whatever deal you might have been offered, unless you explicitly accepted it there and then, becomes null and void. The way that VM run their customer services appears to be that individual agents have a fixed number of 'deals' or total amounts which they can offer to customers, and this will vary from agent to agent. So when you called back, the second agent simply may not have had the ability to offer the same deal (computer says 'no'). It doesn't matter what you may or may not have been offered previously, that's all in the past and gone, VM, or indeed any company and under no obligation to keep it open for you. Legally only if a deal is 'offered' and you agree to it and 'accept' it there and then, is a contract made which is legally binding on both parties. Everything before that is simply a set of negotiations which either party can pull out of when ever they like!
Now having said all of that, I suspect it may have done you a favour anyway. Initially you said about poor quality on video calls to Australia, now for things like this the 'speed' is almost irrelevant, even the 10Mb/s upload speed on VM's lowest normal tariff is more than adequate for a single stream video call. No what is more important it latency, and boosting your speed is likely to do precisely nothing at all to fix that, all that would have happened is that you would have found yourself tied into an new 18 months contract at a higher cost with no improvement in your service.
Golden rule, if you are having issues with your connection, simply upgrading will (almost) NEVER EVER fix it - it will make your wallet a bit lighter but that's about it!
If you care to log into the hub and post up the connection details and logs and also setup a BQM to see if there are any underlaying connectivity issues, then we can take a look and advise accordingly. There are numerous posts on here if you have a look through, which detail how to do these things if you are not sure.
