please note :
ELU29 : extensions will not ring, no ringing generator (REU) in LBP22 possible !!! (only ELU34 allowed)
ELU34 : ringing generator on board.
ELU28 : It might be, that some revisions are not working.
in the documentation "Hardware Status and Reliability" only ELU28 (ELU28 ROF 137 5334/3) is listed.
P.S.
in new magazine LBP22, LBP24 only new HW should be used.
best regards,
Norbert