Cant hear the media playback properly. rtp gets lost

Before asking the question I will try to explain the network situation.

I am connected to a lan through vpn. I have one register server[r1] + one asterisk[a1], running on the other side of the vpn gateway. I am using eyebeam as sip client. the asterisk[a1] plays back a welcome tone when it receives my call. it works fine.

then i installed another asterisk[a2], which is also using the register[r1]. this a2 is on my side of the vpn. both the asterisk a1 and a2 are compiled and installed from the same source version 1.4. also they have the same sip.conf and extensions.conf.

problem is when i call from eyebeam to a2 it shows, in cli, that it is playing welcome message. but the eyebeam does not receive the rtp packets sent from the asterisk. whereas if a1 is called from the same eyebeam the playback message reaches successfully.

can anybody help me? is this problem related to nat or some similar network complications?

please help. thank you.

Note: when i call from one eyebeam to another eyebeam [both in the same side of the vpn, registered to the r1, situated on the other side of the vpn], i can successfully communicate i.e rtp is sent+received properly. [/img]