Mostly see here: http://wiki.voip.ms/article/Cisco_Linksys_PAP2

Also see here: Linksys/Sipura SPA-2100 ATA Connection Notes

If you use a SIP trunk provider that requires 10 digits, modify the Dialplan in the Line1 (or Line2) settings by adding <:520> to the 7-digit section:

|<:520>[2-9]xxxxxx|

In this example, we need 520 (our local area code) added to dial 7-digit calls.

(*xx|[3469]11|0|00|<:520>[2-9]xxxxxx|1xxx[2-9]xxxxxxS0|xxxxxxxxxxxx.)

For maximum bandwidth savings, use g.729a. Otherwise, choose (the default) g.711u (uncompressed) for best audio quality. It's roughly 24Kbps vs. 92Kbps.

Also, be sure to set the RTP Packet Size to 0.020 on the SIP tab.