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The Standard SoundStation2 is an analog phone that sends DTMF Tones to the PABX / PSTN.
If you can use the Phone dialing numbers then the Phone itself works.
The PABX / PSTN System you are using is responsible for passing the DTMF Tones to the far end. You can simply do a test with another analog Phone on the same line calling the same Number to prove that the phone works.
I suggest you liaise with your PABX / PSTN Provider to check the System if it passes DTMF to the far end or if it is a SIP Line if they use the right Payload Type or DTMF Signaling method.
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Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge.
Please also ensure you always check the VoIP , Video Endpoint , Microsoft Voice , PSTN or other FAQ's in the different sections