From: http://toao.net/25-linksys-ata-configuration

Original setting:: Daylight Saving Time Rule:  start=4/1/7;end=10/-1/7;save=1
New Setting:: Daylight Saving Time Rule: start=3/8/7/2:00;end=11/1/7/2:00;save=1

Original Setting:: 
Dial Tone: 350@-19,440@-19;10(*/0/1+2)
Second Dial Tone: 420@-19,520@-19;10(*/0/1+2)
Outside Dial Tone: 420@-16;10(*/0/1)
MWI Dial Tone:	 350@-19,440@-19;2(.1/.1/1+2);10(*/0/1+2)
Cfwd Dial Tone:	350@-19,440@-19;2(.2/.2/1+2);10(*/0/1+2)

New Setting::
Dial Tone: 350@-19,440@-19;30(*/0/1+2)
Second Dial Tone: 420@-19,520@-19;30(*/0/1+2)
Outside Dial Tone: 420@-16;30(*/0/1)
MWI Dial Tone:	 350@-19,440@-19;2(.1/.1/1+2);30(*/0/1+2)
Cfwd Dial Tone:	350@-19,440@-19;2(.2/.2/1+2);30(*/0/1+2)

Also: http://www.voip-info.org/wiki/view/Sipura+Documentation+Line+1

NAT Settings
NAT Mapping Enable : Informs the SPA that it should discover the public address information 
for a unit behind a Network Address Translation router/firewall. The SPA will utilize various 
different methods (STUN/VIA headers/rport headers) to discover what public IP address and port 
is being used during communication.

NAT Keep Alive Enable : Enable/disable support for sending keep-alive packets to keep the pinholes 
poked through the NAT router/firewall from closing.

NAT Keep Alive Msg : Indicate what type of packet will be used for the NAT keep-alive. If the field 
is blank then a carriage-return (ASCII 13) is sent. If the field has $NOTIFY, a SIP NOTIFY message 
is sent. If the file has $REGISTER, a SIP REGISTER without any "Contact:" header is sent.

NAT Keep Alive Dest: Target for where the keep-alive packets should be sent.

From: http://success.ringcentral.com/articles/en_US/RC_Knowledge_Article/How-to-provision-a-Linksys-SPA-2102-ATA

An Analog Telephone Adapter would allow you to use a traditional fax machine or an analog phone 
with your RingCentral service. Follow these steps to provision a Linksys SPA-2102 analog telephone 
adapter. 

From: http://success.ringcentral.com/articles/en_US/RC_Knowledge_Article/How-to-Get-the-SIP-Configuration-needed-for-Manual-Provisioning-2013

The SIP Settings can be obtained from your RingCentral online account to complete manual provisioning of 
an existing phone.  This article will guide you on how to get those information. 

From: http://support-officeathand.att.com/articles/en_US/RC_Knowledge_Article/4743?retURL=%2Fapex%2FATTSupportArtList%3Fc%3DCisco_Phones%26p%3D3%26k%3D%26t%3D%26l%3D%26action%3DSearchAnswer%26pageid%3Dknowledge%26serviceFilter%3D&popup=false&Title=%5BUSER%5D+Setting+up+the+Linksys+SPA-2102+ATA+device+on+Bridge+Mode

Linksys SPA-2102 ATA

The latest firmware allows you to increase the DTMF detection timing so that talkoff (spurious DTMF) 
can pretty much be eliminated. Currently, this can't be done using a PAP2T-NA adapter. This setting is 
under "Admin -> Voice -> Advanced -> Line1/Line2 -> DTMF Tx Strict Hold Off Time".

If your VOIP provider supports it, then T38 faxing works fairly well, with adjustable redundancy levels.