[yfm]

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