According to the ghost attributes in the Fifth Edition System Reference Document, the possessed character may be targeted by the sending spell without any interference from the ghost.
Does the ghost receive/hear it?
Possession (Recharge 6): One Humanoid that the ghost can see... must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is Incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any Attack, spell, or other Effect, except ones that turn Undead... [The ghost] doesn't gain access to the target's knowledge... (SRD page 311)
No, the ghost does not receive or hear the sending message. The ghost's possession ability indicates it cannot access the mind of the possession target. The ghost also cannot be targeted by the sending spell, so its effects would not apply to the ghost.
Does the player receive the sending?
Yes, as the quote above indicates, the target of possession does not lose awareness. They remain a valid target for the sending spell.
Can the player or the ghost reply to it?
If the player casts the sending spell, they can send a message to the possessed character. The possessed character can reply. The ghost is not necessarily aware that the spell was even cast.
If they reply, what voice will my PC hear - the character's or the ghost's?
Sending never specifies what voice the message is sent in.
The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately.
The target recognizes the caster as if it knows the caster. Thus, it would be reasonable to rule that the voices heard in the characters' minds would be like the characters remember each other, not necessarily as they actually sound. For example, if two characters had not met since childhood, their voices might have changed, but the way they remember each other's voices would not have changed to how they actually sound. This is entirely up to the discretion of the DM.