Simple question: Asterisk with Zoiper (no sound)

Hi all:
I’m new with Asterisk and just setup an environment in Vmware workstation (network bridge mode) to test
Windows VM ( Zoiper) ---------- Centos ( Asterisk ) -------------- Windows XP (physical Zoiper)
The only change to the configuration are:

sip.conf:

[jun]
type=friend
username=jun
secret=jun
host=dynamic
context=tutorial

[zhang]
type=friend
username=zhang
secret=zhang
host=dynamic
context=tutorial

extensions.conf
[tutorial]
exten => 1234,1,Dial(SIP/jun)
exten => 5678,1,Dial(SIP/zhang)

1. when call “zhang” from “jun”, the rings sound successfully.
2. when “jun” hold the phone, “zhang” could hear the background music.
3. but when “zhang” or “jun” speak something, the other can not hear any thing.

Could you please give me some guide about it ?

Thanks in advance [/b]