It's not a known issue. Remember that while DTMF events are an out-of-band (or NTE) event on the VoIP side, once the PSTN turns the call into traditional TDM the DTMF events return to an in-band audio signal. At that point anything that can interfere with the call audio (e.g. voltage impeedence mismatch, incorrect input/output attenuation levels, excessive background noise, etc) will also impact the DTMF events.
It's probably a combination of factors such as background noise in the Ops Center as well as something on that particular far-end since this only happen to one dialed number.
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