Century Link provide a Calix 716GE-I ONT to their consumer gigabit (GPON) customers.
This device has a weird 9-pin DIN connector providing standard 12V 2A power. The extra pins are likely used for communication between an external UPS and the ONT, so the device can take actions when the power drops (e.g. prioritize telephone traffic over internet traffic).
However we don't have or want a dedicated UPS for the ONT. We wanted to provide power over the existing ethernet connection. This allows us to piggy-back on the UPS which already powers the PoE switch, and maintain functioning internet (inc wifi) if the power goes out.
We took the power lead from a gigabit PoE splitter, snipped off the end which would normally supply a device with power, and soldered on the 9 pin connector. Helpfully the existing transformer illustrated which pins provided power (1+2 +ve, 8+9 -ve). You could just cut off and reuse the connector from the existing transform, however we wanted to keep the transformer functioning as a backup in case of future PoE problems.
The connector and splitter cost only $25 in total.
Compatible 9 pin DIN connector: https://www.amazon.com/gp/product/B01HJKD1LS/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Gigabit PoE splitter https://www.amazon.com/dp/B00MOIDXZ0/ref=cm_sw_r_tw_dp_U_x_Dv.3Eb3525T24