VOIP doesn't use http to transmit calls. The only thing that port 80 (http) would be used for is for the modem to communicate with the VOIP provider for configuration changes or other information.

VOIP uses SIP and RTP to make a phone call. SIP is responsible for establishing the connection, making the call, and disconnecting the call. RTP is responsible for actually transmitting the voice packets and maintaining a quality connection. When a call is made, a random UDP port between 10000 and 20000 is selected for the call.

Hopefully the stink that they caused last time by blocking VOIP has taught QTEL a lesson about trying it again.