I have been using Linksys SPA 3000 with an Asterisk server with following configuration:

Line1:
--------
Proxy and registration  -
Proxy: 192.168.1.100 (asterisk server)  
Register: YES

Subscriber information -
Display Name:1000
User ID:1000
Password: my_password


When (trying) to register with  FS, my settings are

Line1:
--------
Proxy and registration  -
Proxy: 192.168.1.101 (FS server)  
Register: YES

Subscriber information -
Display Name:1000
User ID:1000
Password: 1234

But.....registration always fails.
I've not been able to find out why.

When I try to register with a softphone (SJphone) with the same User ID (1000) and a Password (1234) then NO problem.

Grds.
Henk