Strange router settings

[#1]

I have just got a replacement router as my plusnet provided one was possibly giving a lot of problems. I have been told that my line looks fine from plusnet's end and with no drop outs but over the last week or so I have had tens per day, sometimes for a few seconds and today went on for quite a while and I had to switch off and restant everything.
I have now got a Billion BiPAC 8800NL router which I understood would replace both the plusnet router and the BT Open DSL modem. I connected everything up and could not get anything to work using the Billion unit alone until I tried the following combination:
Billion unit is connected to the incoming phone line with the RJ11 cable. The BTO modem is connected via an RJ45 cable to one of the 4 ports on the billion router but is not connected to the phone line or anything else except the power.
I have checked all settings and seems correct (username@plusdsl.net, password) and I am using the VDSL setting. It works in this configuration but when I try it without the BTO modem being connected it stops working !
I am totally confused with this, the BTO modem is not connected to anything except power. No other network connections so as far as I can se, is doing nothing so why does the billion router not work without it ? I understood this was a replacement unit for the two units but has defeated me.
Any advice would be gratefully received.
H.

[#2]

there is a thread here  http://community.plus.net/forum/index.php/topic,136198.0.html   it's a few months old but it could be of some use to you, if after your still having trouble maybe post in it and see if someone is still using one that can help.
good luck.

[#3]

Thanks. Will see what I can find.

[#4]

Update.
I just unplugged the BT O modem and switched it off, took the RJ45 cable out so the Billion is now standalone, connected to the phone line only and works !
No idea what is going on. Only thing I can think of is that when I connected it to the router it was at the same split second that the router decided to connect, so could have been a line drop again.
I will just monitor for the next few days and see what happens.