I don't know if there's any information about upstream buffering in the Telo, but perhaps the Telo guys could take a look at this issue in their spare time if they haven't already done so. This would apply when the Telo is placed immediately behind the modem, and all internet traffic from the LAN is going through it. So this issue would affect all traffic, not just phone calls.
If all my traffic is going through the Telo, it would be nice if the Telo behaved well in this regard, including using the buffer management algorithms that have been developed for optimal performance, totally aside from the QoS issues involved with phone calls.
The Wikipedia entry on bufferbloat gives some general information.