The logs show that your fax machine is answering the call, then, roughly 15 seconds later, the call hangs up. This isn't being caused by Google Voice. The fax negotiation is failing. There are several possible causes, all resulting in the inability of the fax modems on either side of the call being unable to set up the "conversation". Your Internet service could be unreliable enough to cause excessive jitter or latency, or the carrier path between the sending fax machine and Google Voice could be degraded, or you've got the machine set up to use fax error correction or a too-high speed.
Try setting the fax to 9600 bps and turn off error correction. If that doesn't work, try sending a fax from some other machine/location. If it still fails, then it's not going to work.