If you're determined not to pay anything for phone service, please ignore the following...
I'm using a free DID I got from Callcentric for their fax reception service. The server is always on, never runs out of paper and just forwards you an email with a pdf of your fax. I pay $1.50/month for E911 service. Incoming service is included on the DID, which by the way is in New York State.
You can use your machine for outgoing faxes. If you shut off its fax reception, you can use it on your existing GV account. If you can't, you could configure SP2 for the Callcentric account and use that for outgoing. Unless you shut off the fax reception via the Callcentric account management web page, that line will never ring but will work fine for outgoing. (Which, by the way, you have to pay by the minute for.)
It also occurs to me, that you could set up SP1 to only ring Phone 1, and you could plug your fax machine into Phone 2 and have that default outgoing to SP1.