Either way, don't be fooled by the QoS features of these devices. For QoS to be effective it must be enforced at every interface between a higher speed network and a lower one - when in-line on your home LAN both sides of the network are most likely the same speed, and so it will not offer any significant benefit over the QoS enforcement that LMCE's core does for VoIP traffic anyway. Moreover, both will not provide the full benefit of QoS unless you setup QoS on your ADSL router as well, anyway!
By all means use this device to connect LMCE's Asterisk system to a POTS provider, but in this configuration QoS is irrelevant, and you don't need to connect the device to the ADSL modem, only to the LMCE network. Or use it as an ATA on the LMCE internal network, to convert an analogue phone into a VoIP phone for LMCE, but in this configuration QoS is even more irrelevant as the core and the modem are doing the prioritisation. If an ATA is what you want, then any old cheap ATA will do for LMCE.
Background: QoS is usually split into 2 parts, marking/colouring and enforcement. The marking process merely uses rules to select particular traffic and set flags the indicate how they should be treated by later devices in the routing path, but doesn't actually perform any prioritisation or other changes to the flows. Enforcement is the actual process of reordering packets to give some priority over others - this can be done based on the flags set during the marking process performed by previous devices (or even the source) or more often based on marking done by the same device as is doing the enforcement.