Cisco SPA303 on Asterisk - question

Hi guys,

I’m somewhat new to Asterisk and am rolling our own VOIP network at a new location for a small business. I’ve bought a Digium A4 card w/echo can to interface with the PSTN.

My question is this:

The SPA303 phones have 3 lit buttons for extensions on them, and for each, the only configured extension shows up on all 3. Are there any ideas or typical configurations here for using the other 2? I would like for users to be able to call each other internally by extension (working in my test bed already), will have the PSTN incoming calls ring all extensions simultaneously (no agent queues, unnecessary for this business), and transfer/conference calls internally and externally.

Thanks for any hints, I’m nearing the end of the first chapter of Asterisk: The Definitive Guide, so I’ve only just covered the basics of extensions and dialplans.