In band DTMF Digits 1 – 9 from a local SIP phone

In band DTMF digits 1 – 9 from a PSTN POTS phone via SkypeIN

In band DTMF Digit “1” from a local SIP phone

In band DTMF digit “1” from a PSTN POTS phone via SkypeIN

Spectrum Analysis of DTMF Digit “1” from a local SIP phone

(696 hz + 1209 hz)

Spectrum Analysis of DTMF digit “1” from a PSTN POTS phone via SkypeIN

(698 hz + 1206 hz)

Table of Peak Frequencies for SIP and SkypeIN DTMF tones

Scenario:

(SIP ATA)-->(sofia)-->(fs)

(POTS)-->(PSTN)-->(skypeIN)-->(skype_client)-->(skypiax)-->(fs)

Conclusion:

Although the DTMF tones are attenuated there does not appear to be any major filtering or distortion of the DTMF tone when transmitted across the Skype network.