Were the frogs raised together from froglets? Has he seen any aggression?

The reason I ask is because species from the tinctorius group are extremely quiet callers. A lot of times, the only way to hear them call is if you stick your head in the tank. He could quite possibly have 2 males and just never heard them call.

If it were 2 females, aggression would be much more prevalent and noticeable and measures should be taken to separate them. This doesnt sound like the case though.

One last possibility is that he DOES in fact have a pair, but if they were raised together from froglets, the female may be unreceptive to the males calling. The male would have hit sexual maturity before the female and started calling before the female if they are the same age. When the female gets used to hearing the male call but isnt sexually mature, they can start to ignore the call and not respond even when they DO become mature enough for breeding.

One way to combat this last situation is to separate the frogs and put them in separate rooms for a couple months feeding normally and misting normally. After some time apart, reintroduction will spur the male to call and the female should be much more receptive to his breeding attempts. Courting should be much more visible if they are a pair.

Hopefully he's just got a sexually inactive pair that needs a little boot in the rump to get breeding!

-Matt