B757-200 CREW FOUND THEIR ENTIRE INTERPHONE SYS WAS INOP AFTER MAINT DEFERRED THE COCKPIT TO GND INTERPHONE SYS ONLY.

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COCKPIT INTERPHONE SYS INOP; DIVERT TO ORD; OVERWT LNDG. ACFT XXX HAD PLACARD INSTALLED SFO 12/THU MEL 23-6B COCKPIT TO GND INOP. UPON COMPLETION OF THE OXYGEN MASK; INTERIOR; GOGGLE CHK ACCOMPLISHED WITH THE PTT SWITCH; GAVE THE EXPECTED CLICK IN THE SPEAKER OF THE OXYGEN MASK MIKE GOING HOT AND COLD. WE COMMENTED THAT THE CHK ALTHOUGH OK WERE QUITE FAINT. WE DETERMINED THIS MAY HAVE BEEN A RESULT OF THE INOP COCKPIT TO GND SYS. WE CONTINUED TO THE RWY AND DEPARTED. AT CRUISE; WE BEGAN TO DISCUSS OUR CONCLUSION ABOUT THE PLACARDED ITEM AFFECTING OTHER INTERPHONE COMPONENTS. AS A RESULT; WE BEGAN TO INVESTIGATE. WE DETERMINED THAT WE HAD GOOD COM TO ATC THROUGH BOTH OXYGEN MASKS ON VHF; BUT WE FOUND WE WERE UNABLE TO COMMUNICATE ON THE INTERPHONE BTWN EACH OTHER; FURTHER; IT TURNED OUT THE INTERPHONE WAS INOP AT ALL MIKE SELECTOR POS REGARDLESS OF THE SWITCHING USED; HANDSET; YOKE PTT. WE WERE ABOUT 325 MI FROM ORD. WE CALLED DISPATCH. HE TALKED TO MAINT CTL. TECHNICIAN WANTED TO TALK DIRECTLY TO US; SO WE SET UP COM THROUGH COMMERCIAL RADIO. DISPATCH; TECHNICIAN CREW DECIDED TO DIVERT TO ORD FOR REPAIRS. LANDED OVERWT AT 205800 LBS. ORD FIXED THE INTERPHONE PROB. ORD COMPLETED THE OVERWT INSPECTION. THE FLT CONTINUED TO DEST.