There is a bug in the SPA50x/SPA51x latest firmware version 7.6.2SR6/7.6.2f.
The SPA with version 7.6.2SR6 dials the number 4161500000 then waits 2 seconds and then sends DTMF begin '0' RTP packet and plays DTMF tone indefinitely. Doesn't send DTMF end '0' RTP packet and doesn't send other DTMF digits.
The same issue with X (wait)
The SPA dials the number 4161500000 then asks confirmation and after confirmation sends DTMF begin '0' RTP packet and plays DTMF tone indefinitely without sending end DTMF and other digits.
All previous versions work as expected, i.e, the SPA sends all DTMF digits.
Looking into debug I found that SPA with previous version logged the next events
This bug is in the latest firmware version in which other bugs were fixed which affect us. The solution is ether find a workaround or Cisco accepts this bug and provides firmware update. Unfortunately we do not have active Cisco support to claim a bug. I will be very grateful if somebody submit a Cisco bug request.