With Grandstreams, you might want to enable symmetric RTP, enable STUN, and enable random port under the SIP settings.

Having more than 1 device behind a NAT is a VERY tricky situation.

For more information on what's going on because of SIP and NAT not playing well together, I would recommend reading: http://www.voip-info.org/wiki/view/NAT+and+VOIP