SPA2102 & SPA942: Incoming calls never get connected

Dear experts, everyone,

My Pbxes setup is pretty simple:
1) One SIP extension 100, one PSTN extension 200
2) Using FreePhoneLine as a trunk for both inbound and outbound
3) Ext 100 is using SPA2102 as home phone device.
4) Imcoming calls are assumed to ring both 100 and 200.

Now Placing outgoing calls are all fine.

The problem is just at incoming calls:

When people call in, both 100 and 200 ring, however only ext 200 (my cell phone) can answer the call but ext 100 can not, even I pick up the phone but the call is never connected while the caller is still hearing the ringing tone, then goes to VM.

I have tested the SPA2102 setup with other SIP services like FreePhoneLine, Callcentric and LocalPhone etc. the SPA2102 is able to answer imcoming calls very well!

I also change the device to Linksys SPA942 but still no luck.

Incoming call problems only occured when the devices are registered as Pbxes' extension.

I have done a lot of googling before I hopelessly draft this post, hope you experts can help solve this issue, thanks a lot!