Virgin Media have a really weird history with VoIP in general. Their SuperHub 1 (after an update back end of 2012) broke VoIP spectacularly for a number of users (lack of registration).
Turns out the SH1 had a SIP ALG which couldn't be disabled and it really messed with the messaging.
Their new SH2 does work nicely, however running the pingtests and (if you can find one) a Broadband Quality Test at various times of the day will give you a good indication of what to expect in terms of latency and jitter.
That being said, a VM Cable BB can (and does) support multiple G.711a/u calls (remember these are approximately 80-100K per call (streaming)) and probably just under double that for G.729A/B (which is between 50-80K per stream).
If you have a SH1, look on the VM Help & Support to turn it to Modem Only mode and grab yourself an ethernet WAN router - something like a Draytek for example, but do check with your chosen provider for compatibility first!
I am NOT affiliated with ANY ISP on thinkbroadband.com, I have used various ISPs and I AM knowledgeable on the subject of broadband.
"Seek and ye shall find"