[asterisk-users] Change Media IP in SDP

[Harel]

Hello List,
I need your help with information going out on my SDP.
Is it possible to update the Media Address on a per-call basis or a per-channel basis?
Reason:
My Asterisk is in a private network and needs to connect to UA on its internal network and also few external networks. One network is public and the others are not public. Between each other the external networks are not routable. Signaling is flowing with no issues because SIP Registers and NAT boxes maintain sessions correctly. The problem is with RTP. After making traces on all possible nodes of this network I clearly found out that the RTP fails because the Asterisk doesn't manage to communicate the correct address to the UAs in the SDP. It will report its internal IP address and the remote UA will try to send its RTP to this address which, of course, will fail miserably. 
Obviously I can't use externaddr or media_address in sip.conf because it will only be good for one network while the other external networks will fail just the same. Same applies for STUN, it will only be good for the network the STUN requests are being sent from. 
On all networks I have fix IP addresses on my side and I fully control a professional security box. 
Asterisk is 13.6.0
I can't, and don't want to, touch user-side equipment which is normally some kind of voip phone behind a standard home VDSL router.

Any ideas how can I transmit the correct IP address in SDP to UAs on different networks?

Many thanks,
Harel