[Serusers] snom 100 & SER strange behaviour

Hi, I don't know if this is the right place to post this, but I noticed a strange behaviour with the snom 100 with ser 0.8.11 and a grandstream. The snom is behind NAT, but ser is on a public address, the grandstream is on another network also behind nat. The nat is sip aware (cisco soho91). When I place a call from the snom to another IP phone, grandstream, the call doesn't always complete successfully: in 50% of cases after the other end answers the call, I can hear the other party, but they cannot hear me. When I look into ethereal, I find the regular SIP INVITE and rining messages, then I see a bunch of UDP packets (the RTP session) from the other end, the grandstream, but I don't see a stream of RTP from the snom to the other end. Now what is strange here is that sometimes everything works fine, and we can both hear each others, and I do then see the RTP stream in both directions. Both devices share the same codec, and the SDP info is ok, the ports numbers are valid. I don't think this is ser related, but I wonder if somebody has experienced such a behaviour. Thanks. Samy.