Unable to get call waiting to wait

[Syd Carter]

I wonder if someone can help me to get my call waiting to wait.  I'm pretty 
sure it is enabled, according to asterisk database.  When a call comes in 
while I'm talking to another party, the callers id info flashes up and there 
is an accompanying beep.  Unfortunately I think that there is a timeout or a 
reaction to the phone stating that it is busy because the caller gets 
transferred to v-mail within 5 seconds.  I'm pretty sure my settings are 
good, and I use a dial string such as 
"Dial(local/[EMAIL PROTECTED],45,mtTwW)"  Any suggestions?  Thanks..