Dialout from a single line

I would like to connect a caller to a cellphone after he/she hears greetings. This would be on a single line, like Zap/1. Is the second phone line necessary, or is there a workaround through the dialplan:

exten => s,1,Answer()
exten => s,2,Playback(some-greeting) ;"press 1 to talk to Sam"
exten => 1,3,Dial(Zap/1/sams-cellphone-number,20,r) ;this is where 
;Asterisk reports channel is busy, so I tried an alternative:
exten =>1,3,Hangup()
exten =>s,4,Dial(Zap/1/sams-cellphone-number,20,r) ;but sam is not dialed 

I am using X100P card on a single home phone line with Asterisk 1.2.9. Is the second phone line necessary to get the cellphone dialed?

I know on a sip account you can use another channel on the same account. It might be possible to use the other channel it uses for call-waiting and three way calling, in this case you can try:

exten => s,1,Answer()
exten => s,2,Playback(some-greeting) ;"press 1 to talk to Sam"
exten => s,3,Dial(Zap/1/sams-cellphone-number@outboundtrunknamehere,20,r) [size=75]<–you need to define what channel or outbound trunk to dial through after the @ sign[/size]
exten =>s,4,Hangup()

Sorry but Asterisk will not convert your single line into two lines…

Go get a VoIP acount some place, drop 50.00 on account to get outbound calls (Connect.VoicePulse.com and get unlimited inbound 11.00 a month will half a cent a min in most of the 48 us You do not need to get the DID you can just get the outbound mins)

use the Voip outbound trunk as the second trunk (when the 1 is busy it will kick in) if you half CFOB (call forward on busy) on your pots line point it to the VoIP DID. With 11.00 a month with CVP you get four channels for outbound / inbound calls…

So when you have a home full of family hanging around for DAYS on end, or your small business has growning pains you have PLENTY of phones line to use.