That phone contains both a speaker and microphone for full duplex operation. The "S" is the designation for that. If the phone is just stamped as RP624 is only has a speaker for one-way operation. If these phones are placed in noisy environments the background noise can override the speaker operation. They are recommended for use in a quiet environment.
I know the classic rolmphones are definitely not full duplex, and I just did a rudimentary test on the 624 on my desk by calling voicemail and making noise, which caused the speaker to cut out so I assume the 600 series is not full duplex either.
I am pretty sure the optisets are, and anything that came after them (410 series, 420 series, Openstage - not sure about the 500's but would assume they are because the optisets are).
There is an Optiset conference phone.
You will know it is different because it has a small rectangular opening for the microphone on the bottom right hand side.
Here is an example of one
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