SIP(express talk) help

i download a express talk and install it.i want use it through my asterisk.
i am a newer to sip. i config as follows:
first i config my express talk using its setting:
display name :88201234
sipnumber :88201234
server : my asterisk host ip
password : 1234
sip.conf
[88201234]
username=88201234
type=friend
host=dynamic
context=sip-call
secret=1234

extensionf.conf
[sip-call]
exten=>5555,1,answer
exten=>5555,2,playback(demo-instruct)

when i dial 5555 ,i hear the tone but after 2 or 3 seconds it hang up.
what 's wrong?

Upon readin your post (again) I saw that everything looks fine in the setup of Express Talk and the sip.conf file. Possibly your extensions.conf is a little off.

Use this page to double check your Express Talk settings…
http://www.asteriskguru.com/tutorials/express_talk_sip_softphone.html

I have my extension information setup within the [default] context instead of using the custom line context=sip-call line.

Good luck…!

i download another sip client wengo they can work.
maybe setting of express talk is not correct.
any way thank Aaron.