hi felim
who is your isp? If they do not trust the websites results then your going to have go even more basic. Forget specific voip testing. VoIP usually degrades and fails due to packet loss and high latency. The easiest way to test this is just use ping. Make yourself an excel spreadsheet and record around 1 weeks worth of tests, do these at different times of day, eg 8.00am, 18.00, 23.00.
To ensure they cannot refute any of your information, start your pings to heanet.ie - they have 166Mbit/s links so your ISP cannot complain that the bottleneck is with their network.
Eg. ping -n 2000
www.heanet.ie (this will send 2000 icmp requests)
at the end of the test record, the packets sent / received, lost and average RTT
Hope that helps