Can`t make a call from one direction, but can do that from another one

Hi everyone. I have the next scheme: VPS Server (S), which contains Asterisk and OpenVPN Server, phones (A,B,C) which are connected to “S” as ovpn clients and connected to Asterisk on its local ovpn address, sip phone (D) which is located in the office. Server “S” is also openvpn client of a router ® in the office, so, phone “D” is connected to “S” on openvpn address too. “S” as openvpn server and phones A,B,C have, for example, addresses in net, “S” as a openvpn client and office router have addresses in net, and phone “D” has address All routes are correct, and I can ping and trace “D” from “S” and even from “A,B,C”. Can ping and trace Asterisk address from phone “D” net.
The problem is: I can`t make a call from phones A, B and C on phone D because of “Retransmission timeout reached on transmission”. I hane no NAT in my local net and the problem exists even when I open firewall on “R” and “S”. But I can make calls in opposite direction - from phone D to phones A,B,C. And also I can make calls between A, B & C without any problems.
Can anyone help me to solve this issue, or just suggest where to dig.
If it is necessary to post configs or debug reports - I will do it.
Will be thankfull for any help

Dig by obtaining protocol traces.