I have Via Talk (VT) VOIP service.  The service I use includes 2 phone
numbers -- one is a fax line, one is a voice line.  The voice line can
handle 2 calls -- any combination of outbound and inbound.  However, I
only use it for outbound calls.  I never use the fax line.

My phone software is Asterisk-based -- SwitchVox is the name of it.

I don't need the fax line.  I would like to have more outbound lines. 
Is it possible to utilize the unused fax line for additional outbound
calls?

Bonus: any alternatives to ViaTalk for unlimited monthly minutes of
business calls?  The more lines out, the better, however, we usually
have only 1-2 going out at a time, rarely sometimes up to 5 out at a
time.  With Junction Networks we can have as many users as we want
making outgoing calls, but each user is charged 2.9 cents/minute, even
toll-free calls (JN quality is very good).  Right now we use VT as our
first shot and use JN as our "overflow" provider when the 2 VT lines
as tied up.

Asterisk has the capabilty to have multiple outbound lines accessed
via dial out rules.  I'm familiar with Trixbox, but not SwitchVox. 
There's various web resources that describe tihs.  (Namely for
businesses who have multiple VOIP providers with different
day/night/international/domestic rates.  There are ways of selecting
the best VOIP provider based on these rates so that you get the lowest
cost phone).  Your cases should be a simple subset of this.