As in Gateon Port, there are 4 Pokémon in Phenac City who have lost the ability to speak

Pokémon 1
The first likely Pokémon you will see once you enter Phenac is the Castform that runs around the fountain with his trainer, unfortunately it too has lost it's voice. Talk to it and the Decoder will bring up a question. Below is the question and the correct answer

Q - What kind of the Pokémon is this?
A - The weather Pokémon
Pokémon 2
The next Pokémon is the Azumarill that stands in front of the Pokémon Center. Talk to it and the Decoder will bring up a question. Below is the question and the correct answer

Q - What is this Pokémon’s pre-evolved form?
A - Marill
Pokémon 3
The next stop will be near the Right Hand exit of the city, the trainer and his Linoone. Talk to it and the Decoder will bring up a question. However this question is an Audio Question and it will ask you for Linoone's Cry

Q - Which is this Pokémon’s cry?
A - the second one
Pokémon 4
The final Pokémon in Phenac City is the Seedot in front of the Stadium. Talk to it and the Decoder will bring up a question. Below is the question and the correct answer

Q - What is this Pokémon?
A - Seedot

With the Final Pokémon done, it just leaves the Wobbuffet in Agate Village so head to Agate