I just installed the latest FreePBX Distro from Schmooze FreePBX-2.210.62-5-x86_64-Full-1359059426.iso.
When I tried to provision my LinkSys SPA921 it failed with tftpd: read(ack): Connection refused
What I found was that the SPA921.cfg file was 64k as generated by the OOS Endpoint Configuration. This is larger than the TFTP buffer in the SPA921. I have the Profile compiler from Cisco so I made a full sample profile XML file and found it to be only 42k.
The configuration file produced by OOS Endpoint Configuration is pretty with nice 8 space indents and lots of comments. The Cisco compiler uses 2 space indents and section comments.
The config file is for the entire SPA9XX line of phones so it has provisions for up to 6 lines. Since the SPA921 only supports one line I deleted the configuration for lines 2 through 6. This made the the file small enough to load.