Problem registering ATA and soft phone together
 

I am able to register both lines of my SPA2102 ATA and received calls are directed to both lines. But when I start the X-Lite soft phone from my computer, it breaks things. Either the soft phone doesn't receive calls and the ATA's two lines do, or vise versa. When the soft phone receives the calls, but not the ATA, the ATA thinks that both lines are registered. Even after I shut down the soft phone, the problem persists - incoming calls are not received by the ATA. This issue does not affect outgoing calls - they work fine from the soft phone and the two ATA lines. Has anyone else experienced this kind of problem? Any suggestions for changing a setting to correct this?

Just use different SIP source ports on SPA/Line1, SPA/Line2 and X-Lite.
Something like 5062, 5064, 5066 respectively.

Probably the SPA2102 and the SIP softphone are using the same SIP and RTP ports.

Make sure that every SIP device behind the same firewall is using a different SIP and RTP ports. Place RTP ports at least 2 ports apart (or use only even numbered ports).

Thanks for the inputs! I had allocated different SIP ports (e.g. 5063 and 5064 for the ATA lines), but I have not changed the RTP ones. I think I can figure out how to do this for the ATA, but I'm not sure how to do this for X-Lite. If I could learn its default value, then I could work around it with the ATA assignments.

Checked X-Lite and could not find a field for the the RTP/RTCP entries.
The default is usually 5004/5005.
Probably the easiest thing to try is to use 5006 and 5008 in your ATA for the RTP entries to minimize the likelihood of conflicts.
You may also want to forward the corresponding port range (5006-5009 UDP) to the ATA'a IP address if you get one way or no audio.

There is no need to touch RTP ports at all.
Linksys boxes & X-Lite/eyeBeam/Bria are using non-overlapping port ranges for RTP (by default).

There is no need in port forwarding. If you will have one-way audio issues you will need to configure your adapter properly. Please let me know if you need any help with this.

Hmm - if the RTP port assignment isn't the problem, and if the SIP ports aren't overlapping, any ideas what else could be causing these symptoms?

It's hard to guess without having real information from adapter and possibly router.