B777 Captain reported after takeoff experiencing flight and cabin inter-phone failure resulting in the flight crew diverting to an alternate airport where they landed safely.

Narrative

After taking off from ZZZ; we noticed several issues with flight interphone and cabin interphone. Both were inoperative. There was no way to communicate with the cabin. The relief pilots remained in the flight deck as we discussed options. We decided to make a PA informing the flight attendants of the issue and for the purser to set up and come to the flight deck. On the cabin cameras we observed the flight attendants properly following flight deck access procedures. Once the purser was in the flight deck; we informed her of the situation. I used captains authority to authorize the use of texting between her and us to communicate; and informed her we were going to try and troubleshoot the issue. Dispatch and Maintenance Control were contacted. After 20 minutes of unsuccessful troubleshooting; it was determined that the interphone was not going to be operative. A discussion occurred between the crew and also the dispatcher. Due to the possible safety issues involved with not being able to communicate with the cabin crew in normal or emergency situations; we decided the safest course of action was to divert. After further discussion with the dispatcher; ZZZ1 was determined to be the best option. Due to our weight; fuel jettisoning was required to get below maximum landing weight. ATC and dispatcher were notified; and fuel jettisoning was accomplished. The cabin crew was informed via text and the passengers via the PA. The FOM divert guide was reviewed and an uneventful landing in ZZZ1 was accomplished. Reports were also filed for the diversion and fuel jettison. On landing in ZZZ1 our pairing showed our crew continuing after a 2-3 hour sit and a plane change. I was conducting the first leg of a captain OE (Operational Evaluation). After talking to the OE pilot; I determined that due to the previous flights irregular operation and the possible very long duty day ahead of us; proper and beneficial training would not occur. I made the decision to terminate his OE in ZZZ1. I informed the crew desk that he would not be continuing his training that night. They asked if I was calling in fatigued and I said no. After some discussion with the Chief Pilot the crew desk removed us from the ZZZ1-ZZZZ flight.