You have to provide more details about the sender SMS app, device. Is the sender's number a PSTN mobile number? Is your number a RingCentral number? Does the sender receive group message or one-on-one? If he/she receives messages as a group message but when he/she replies to the group, it ends up in one-on-one message for all, then the problem is most probably on that member SMS app settings.
If this happens on the RC app, then let me know.
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
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