I agree, it's not necessarily the ATA's fault. My point was: the extra tones are the CAS, SAS, ACK associated with CWCID.
...and, I never had this problem with POTS using these exact same phones. VoIP is the x-factor, and the ATA is a logical suspect (but not the only suspect, obviously).
Something I just found in a firmware release notes document for another Linksys device (resolved issues):
"CIDCW Ac digit (A or D) is sent out-of-band if the DTMF Tx Method is INFO. The symptom is that the other party may hear the ACK digit if the peer device plays out-of-band DTMF digits via a SIP INFO message"
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