Remember that these devices use pins 2&5 of a 6-pin jack (3&6 of a 8-pin) and are polarity sensitive. It is not uncommon to find 2-conductor line cords, these phones will NOT work with those, they must be 4 or 6-pin line cords and it must be a "reversed cord" which is a standard telephony line cord, and the polarity must be correct at the jack.
If all these requirements are met, then the phone will come online on it's own in almost every case.