At least we think it is tesltra clear with the issues.

We are with Vadacom, who use 2talk.  We have two phone lines going through the vadacom cloud system.  Which basically means we plug the preconfigured phones into the net, and we are away.

However we can rarely get more than one line at a time.  If my boss is on a call I can't make a call, and incoming calls, while ringing, just can't be answered, they sit on the line and we can't hear anything.

This is the case maybe 8 out of 10 calls, so very occasionally we do have two lines.

We have tried three different modems, Draytek Vigor 2750n, a gigabit something or other and a netcom something something twinned with a thompson.  

We have a VDSL line in takapuna.  Bizzarely enough we get the best phone connection when just using an adsl2 modem.  The Draytek is VDSL, but when using that, or the thompson/netcom combo outoging calls will just fail, and sometimes an incoming call with disconnect an existing call.

On the ADSL2 we just have the original problems mentioned above.

NONE of the modems have SIP ALG enabled, and even tellnetted into the draytek to disable it, the other two don't even have the options in there.

Vadacom/2talk are sure it isn't them, and Telstra have no idea what is going on, but the IT guy thinks it might be a firewall setting at Telstra's end.

Any ideas or people with similar issues?