http://www.dslreports.com/forum/remark,13657675
And now (drum roll), how to forward all inbound calls to the OTHER line:
This is VERY USEFUL, because it either lets you have a TWO VoIP accounts that both “ring” the same phone, OR lets you use one account for all incoming, and a 2nd account for all outgoing (by putting the “phone” on the line with the outgoing VoIP service, and then forwarding all incoming calls on that other VoIP line to that one)!

NOTE: This theory was tested earlier this evening, by forwarding my SPA-2000’s “Line 1” (setup for FWD) to “Line 2” (setup for DialPad.com), and then calling my FWD number from Packet8. After I finally got all the pieces in place, my “Line 2” was happy to “ring”, and when I picked up that phone 2-way talking worked fine! So this appears to work (at least for me). But naturally YMMV.

Here are the needed pieces:

1) As in the previous “trick”, you need unique SIP ports and unique userids for the two lines. NOTE: It’s quite OK to use whatever “userid” the provider on that line supplied (for logging into their SIP proxy). You don’t need the UserId set to any specific value, just something unique!

2) Again, the line you are forwarding from will need “Make call without reg: Yes”, and the line you want to forward to will need “Ans Call Without Reg: Yes”.

3) If you are behind a router (I am), you will need to forward the SIP port of the line you want to ring (the line you are forwarding to) to the SPA. This is probably much easier if you program the SPA for a “static LAN IP” (instead of using DHCP).

4) Your external address will need to either be “static”, _OR_ you will need to use a dynamic DNS service (btw: I’m happy with the free dynamic DNS service from Ã