Not sure if they're outdated from a patch or just flat-out wrong, but both wikis say that phantoms cannot use undead rapport:
Wikispaces - Undead Rapport
Wikidot - Undead Rapport
As a summoned phantom in The Depths, I successfully used undead rapport on a torch-wielding hollow. It killed another hollow and then got killed by a slime.
Wikispaces - Undead Rapport
Wikidot - Undead Rapport
As a summoned phantom in The Depths, I successfully used undead rapport on a torch-wielding hollow. It killed another hollow and then got killed by a slime.