Should be easy to fix!
The configuration in zoiper for the DTMF (keypress events) does not seem to match the configuration used on the server.
There are 3 types of options:
RFC2833 (the keypresses are sent with the media (RTP) as text)
SIP INFO (the keypresses are sent with the signalling (SIP)
Inband (the keypresses are converted to tones and sent as audio (RTP) - awful, doesn't work with compressed codecs, has false positives, could be picked up double because of local playback... by all means avoid this codec unless you really like to live dangerously.
So in short, go to the zoiper settings -> accounts -> "your account" -> features and try changing the DTMF style to one of the first 2. (or the inband as last resort).
The keypresses should now work!