Ok, went through the nvm settings for the c405 I thought would apply, that also weren't invalid, on the 6515.
The one I thought would do it was changjng the line type for Ch0 from PBX (value 1) to PSTN (value 0), nvm 823-036, but still wakes up every 30 minutes with the VOIP phone line plugged in. The other line types for the other channels are already PSTN.
I also changed the values of "fax card hook detection threshold voltage" values up and down 2 for 826-004, 005 and 006 (6, 10 and 20 respectively) just in case the voip line voltage was changing during those episodes, triggering the wake up. I didn't want to go too far from default.
I don't fax all that often but would like to have this work properly.