tlphn xchng, ls knwn s tlphn swtch r cntrl ffc, s crcl cmpnnt n th pblc swtchd tlphn ntwrk (PSTN) r lrg ntrprs tlcmmnctns systms. t fcltts th ntrcnnctn f tlphn sbscrbr lns r dgtl systm vrtl crcts, nblng tlphn clls btwn sbscrbrs.
Th trmnlgy sd n tlcmmnctns hs vlvd vr tm, wth tlphn xchng nd cntrl ffc ftn sd ntrchngbly, th lttr trm rgntng frm th Bll Systm. cntrl ffc typclly rfrs t fclty tht hss th nsd plnt qpmnt fr n r svrl tlphn xchngs, ch ctrng t spcfc ggrphcl rgn. Ths rgn s smtms knwn s th xchng r. n Nrth mrc, th trm wr cntr my b sd t dnt cntrl ffc lctn, ndctng fclty tht prvds tlphn wth dl tn. Tlcmmnctn crrrs ls dfn rt cntrs fr bsnss nd bllng prpss, whch n lrg cts, mght ncmpss clstrs f cntrl ffcs t spcfy ggrphc lctns fr dstnc msrmnt clcltns.
n th 1940s, th Bll Systm n th ntd Stts nd Cnd ntrdcd ntnwd nmbrng systm tht dntfd cntrl ffcs wth nq thr-dgt cd, lng wth thr-dgt nmbrng pln r cd (NP cd r r cd), mkng cntrl ffc cds dstnctv wthn ch nmbrng pln r. Ths cds srvd s prfxs n sbscrbr tlphn nmbrs. Th md-20th cntry sw smlr rgnztnl ffrts n tlphn ntwrks glblly, prplld by th dvnt f ntrntnl nd trnscnc tlphn trnks nd drct cstmr dlng.
Fr crprt r ntrprs pplctns, prvt tlphn xchng s trmd prvt brnch xchng (PBX), whch cnncts t th pblc swtchd tlphn ntwrk. PBX srvs n rgnztn's tlphns nd ny prvt lsd ln crcts, typclly sttd n lrg ffc spcs r rgnztnl cmpss. Smllr stps mght s PBX r ky tlphn systm mngd by rcptnst, ctrng t th tlcmmnctn nds f th ntrprs.