RTP does not pass between Gigaset C530A IP and Asterisk. What will be the ideas?

Question

Asterisk

There is a Mikotik RB750 that listens on ports and redirects them to a server with native Asterisk 13.5.
There is a remote point where there is a Vogtek phone (in Russia it is SNR) connected to Asterisk. There are calls back and forth, RTP passes. There, on the remote side, a DSL router and NAT.
They set up a remote point as separate numbers and sent a Gigaset C530A IP with an additional handset. If you call them, then everything is fine - RTP goes.
If you call from gigasets, then the connection is established, but RTP does not go through from there, and they do not receive it from Asterisk.
Need ideas. There are no problems with Vogtek. SIP accounts are configured identically.
PS sometimes there are "successful calls" from a remote point - RTP goes back and forth. But rarely, and under what conditions is unclear.

Answer 1

You understand that there is little information .... but in short:
1. "Smell" the traffic on Mikrotik; it will be clear what goes where and why it does not go.
2. Temporarily disable all blocking rules on Mikrotik.
3. Play around with SIP ALG in Mikrotik (well, DSL router)

Answer 2

Alexander, make tcpdump on Asterisk and watch SDP. It is likely that Asterisk does not always get the correct address from the seed side.
If at random, try enabling / configuring STUN on Siemens and test it.

Answer 3

A typical problem that we face when using Gigaset handsets is that very often their packets to Asterisk come through a tunnel with the external ip-address of the remote router in the header, you can see the connections in the conntrack table.
Almost always, the solution is to reject packets towards the external interface on the remote router from the range of phone addresses.