We had the NBN connected just before xmas - and it's FTTH!

It started out going well except the phone went dead - no dial tone - every now and then. Recently the frequency of dropouts has increased and we have even had a dropout occur during a call.
The problem only affects the VoIP service; I can browse the internet while there is no dial tone. I've removed the splitter and second phone and checked my wiring. I've monitored the router via it's web interface and I cannot see any difference before and after dropouts. All the lights on the router stay green. Rebooting the router fixes the problem.
Before I spend half an hour listening to my telco's recorded 'music' can anyone enlighten me as to what is happening? If it's my problem I can fix it and if I do have to call them I'll be in a better position to get a solution if I understand the problem.
Thanks in advance.