Sometimes you may have one way audio (one way RTP) problem on sipXecs phones.
Correct functioning sipXecs registered phones behind of NAT requires ports forwarding.
Or if network has firewall next ports should be opened:
5060 TCP and UDP 5080 TCP and UDP 10000 - 31000 UDP (for RTP) Also on some modems should be enabled sip-passthrough mode. See Netopia, Motorola..