Nokia vs tivi SIP clients

I am making VOIP calls over 3G using Asterisk between two Nokia N80. I capture RTP packets at one of the terminals. When I use Nokia SIP client the packet loss rate is much higher than when I use Viti SIP client. Other difference is that Nokia client is sending double number of packets for the same speech and codec. Can anyone tell me why different loss rate and number of packets when same speech and codec is used?