Steve,
This problem is not that un-common and is not just effected by broadband speed upgrade, which, like many other things does effect the line spec. You will also be surprised to note that this is also effected by wet weather as well.
When your broadband was upgraded, a board was replaced in the exchange and whilst it is almost certainly going to be within the required spec, it has provided a situation where the threshold for hangup has a small difference which causes your problem.
There is also another common situation caused by the same issue and that is where someone leaves a message, but the message indicator does not show a new is waiting, so there is no reason to press the News button and so you miss messages left.
I suggest you call your Telecom supplier and report it as a line problem and go from there. It is likely that there first feedback will be that there is nothing wrong with the phone line, then you will have to push back and get them to change the line driver card anyway.
Also, whilst you have stated that you have changed the microfilter, be aware that there is big differences between microfilters in these situation and you may wish to try a higher spec'ed version. You may wish to look at professional microfilters HERE