Incoming calls not received
 

I have a problem that I am sure is related to my network and not Voxalot. Yet I am seeking your help on what I should do to sort it out. My device is GrandStream H502. Here are my settings:
Advance Setting:
No STUN server used.

FXS Port1:
Primary SIP server: voxalot.com:443
Outbound Proxy: us.voxalot.com
SIP Transport: UDP
NAT Traversal (STUN): Yes
Local SIP Port: 443
SIP T1 Timeout: 2 sec

The rest is the default setting of the GrandStream

Here is additional information:
Product model: HT-502 V1.2A
Software Version:
Program-- 1.0.1.41 Bootloader-- 1.0.0.9 Core-- 1.0.0.37 Base-- 1.0.0.87

The problem is:
1) The ATA gets registered with no problem.
2) I can make outgoing calls with no issue.
3) The problem is in the incoming calls. Callers when dialing my number they receive no ringing tone. Yet, my telephone rings and I even see their Voxalot ID. When I answer, I receive a busy signal.

As I mentioned above, I am sure this is a problem specific to my network because when I travel and uses unrestricted networks, everything works fine.

My questions are:
1) What would I need to look at in my settings or what is need to be changed to solve this problem?
2) If I use a STUN will it solve the problem? What STUN I should use and what are the settings?
3) If nothing to be done in my settings, what would I need to ask the System Admin of my network to do so the problem is solved taking into consideration the following:
a. I cannot have a fully unrestricted network. I may convince them to tweak it as long it does not form a security issue.
b. Our system admin has limited experience and I have to give him a specific and precise request.

Regards
Y

I never had a possibility to test a GrandStream, but all hardware I tested until now are quite similar.
For now, just some items:
1) You set "true" to "NAT Traversal (STUN)" but did not configure the STUN server according to end of first paragraph. I would say: If your admin is not so skilled, you may have chance to be in a non-symmetric NAT and then use STUN.
So, use the STUN server stun.voxalot.com

2) Don't use outbound proxy if it is not really needed. But If you telle that outbound calls are ok, don't change it for now.

3) Use us.voxalot.com as SIP/REGISTRAR (or "Proxy" according to some hardware, but this is not the same as "outbound proxy"). I got similar issues when I misspelled the server to "voxalot.com" in place of "eu.voxalot.com". Everything was ok for outbound, but I was unreachable for inbound.

If you are not in a symmetric (or NAPT) network but just a "simple" one, it should work just fine with STUN. (No need to open ports, so no security risk on this side.)