[Asterisk-Users] about dial parameter L

[FCG ZHAO Zigang]

quesion 1:

If I write extension.conf

exten => 100,1,dial(sip/100,20,L(0))

I will listen ring but I don't access when other one dial 100.

who have this experience ? 

quesion 2:

and I look at this extension.conf in voip-info.org

exten => _908.,1,Dial(Modem/ttyI0:${EXTEN:1}) 

Could this modem dial to PSTN?
and will I install dial program for the modem? 

and when I write :

exten => _908.,1,dial(modem/ttyl0:${EXTEN:1},20,L(0))

I will dial to PSTN, but dialee will only ring.

then for me,if dialee access,will I pay phone rate?
who have experience? help me. 
Thanks.