DTMF tones are not sent during call
While in a call, it is not possible to use the dialer-app's keypad to send DTMF tones. This means that many voicemail services or meeting dial-in services are unavailable to PinePhone users.
It appears that this is an issue in oFono, since it handles asking the modem to send a DTMF tone. We also have a local tone-generator which generates these sounds, but this is only done for the local user so they can hear the tone too.