I have quite a long winded question,

We have a customer with around 40 sites, we are going through the process of installing adsl2 services at each site purely for voice,(Sip trunks via engin).
This particular customer has a managed VPN which we were considering trying to use for the purpose of SPNET calls.
Our problem is the telephone network and computer network are seperated, is it worth the trouble of setting up our routers to send SPNET traffic through their VPN Network, or is it just going to create headaches??.
We are only wanting to use the VPN to take some pressure off the adsl2 services.

The other thing to consider here is that if you plan to run SPNET over the Engin DSL then you can't use the voice only adsl services, hence a higher cost for the adsl.

Do you plan to use the Engin supplied SRP527W's or your own router?

Depending on how the networks are setup (physical seperation or vlan) will depend on what you need todo to enable the spnet via their data network (this also brings into play things like QoS)

In theory we need to upgrade the upgrade the DSL, but 9 times out of 10 the SPNET works.
Yes we are planning to use the SRP527W router. As it is at the moment the networks are physical seperated, most sites we just have our telephone system connnectly directly to the modem and in cases of IP phones we have our own POE switch.