Hello, 

Tyler from RingCentral support here. 

This is indeed a strange occurrence and not one I have personally seen before.

The best thing to do would be to open a support case with detailed information about the SMS messages and phone calls your client received (I have pasted some templates below). If you can recreate the issue, at least 2 examples from the last 24-48 hours are essential if we need to open a ticket with another carrier. 

Without seeing the data I can only speculate: my best guess is somewhere along the line between your RingCentral endpoint that is sending the SMS and your client's endpoint on the receiving end the SMS is being re-interpreted as a "call" invite causing the far end to ring in error. The path might look something like RingCentral    intermediary carrier     intermediary carrier   your client's endpoint carrier. 

SMS Example 1:
Number (From):
Number (To):
Date:
Time+Timezone:
Result: "A user of ours tried to send a text message to a client, the client didn't respond, later when he spoke to the client, the client asked him why he called him so many times, so he realized that for every text message he got a phone call instead of the text message"

SMS Example 2:
Number (From):
Number (To):
Date:
Time+Timezone:
Result: ditto 

Call Example 1:
Number (From):
Number (To):
Date:
Time+Timezone:
Result: ditto 

Call Example 2:
Number (From):
Number (To):
Date:
Time+Timezone:
Result: ditto

I hope that helps, please let us know if you have any additional questions or concerns. 

All the best,  

Tyler Dickey | Tier II Technical Support EngineerEMEA
Global Services & Solutions
Connect with me:
888-898-4591
RingCentral
support.ringcentral.co.uk

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