Here is the list of SIP Trunking providers that I know I have had workings with.

The good:

XO

Global Crossing

So far, I have personally tested both providers, and get a good success rate. 

The Bad:

Level3

We have used Level3 for years (since the broadview days) and have never gotten outbound faxing to work.  Inbound faxing has a high success rate for our company.  With that said, we also have not tested outbound faxing in years through that circuit as well.

The Ugly:

Verizon

I have been hearing for years (since 2008) that verizon was going to support T.38 in the next couple of months.  Fast forward to today, and they are still saying that they will support T.38 in the next couple of months.  The takeaway?  If you want to FAX over SIP Trunks, do NOT use Verizon!