The find familiar spell says (emphasis mine):

While your familiar is within 100 feet of you, you can communicate with it telepathically.

So, clearly you can speak to it telepathically, say to issue commands to it. But can it:

  • Reply to your message?
  • Initiate a conversation with you?

As it happens, I'm going to have a pseudodragon familiar (via warlock, Pact of the Chain), which can respond in it's own way:

Limited Telepathy. The pseudodragon can magically communicate simple ideas, emotions, and images telepathically with any creature within 100 feet of it that can understand a language.

So in my case, it's more about whether it can reply with words (either because it can speak to me telepathically due to find familiar, or because I've "initiated" a telepathic conversation with it, similar to the message cantrip, even though that's not actually telepathy), or whether it can only reply at all because of its own ability (via "Limited Telepathy", which obviously won't be with words, if it turns out that the telepathy via find familiar is one-way only).

I'm hoping to figure out which interpretation is correct by knowing the answer to my above questions about whether any familiar (via find familiar) can telepathically communicate with the caster at all.