Farallon, in its PhoneNET connector documentation, recommends that users not
connect a LocalTalk connector directly to a twisted-pair backbone via a
PhoneNET to LocalTalk adapter cable. The reason is that LocalTalk connectors
don't have sufficient surge protection when connected directly to the phone
line. The documentation says that the first connector out of the wall
should be a PhoneNET connector. Then, it's safe to branch LocalTalk from
that using the adapter cable.
Farallon incorporates a proprietary transformer and surge protection system
into their connector modules. They do this because phone wire is not
shielded and is subjected to RFI and EMI. Because LocalTalk cable is
shielded, the same level of surge protection is not required in LocalTalk
connectors. Connecting LocalTalk cable to PhoneNET via an adapter is not
recommended, because it reduces the shielding effect at the cable. There
are other reasons, including differences in impedance. Farallon's suggestion
correctly interfaces phone wire to LocalTalk with a module that provides
some form of surge suppression.