The only way you can get an echo is if you send the audio of the caller back to the caller.

Without knowing the actual specifics of your setup, I can't say for sure how you're doing that, but my guess is that it's the "speakers in the studio, so the announcer can communicate with the guest".

Using a speaker in conjunction with an open microphone, unless you really need to, is unnecessary at best & problematic at worst.

As for how to fix it, assuming that is the problem:

If you don't need speakers, ditch them.

If you do need speakers, because there's an audience (that's essentially the only situation in which you need speakers), get an extremely directional microphone, keep the microphone behind the speakers (and facing the opposite direction), treat the walls with sound absorbing material to reduce as much audio bouncing as possible, keep the gain on the microphone as low as possible & use a noise gate, and EQ out whatever ends up finding its way back into the mic.