DTMF with Nokia E71 running firmware version 500.21.009 was impossible to
444@sip1.budgetphone.nl (account balance) IVR.
Only when changing Voxalot's Voice Service Provider for Budgetphone, "Optimize Audio Path" from "Yes" to "No", made DTMF digits arrive at the remote IVR application.
Setting from "Optimize Audio Path" from "No" to "Yes", and the next call DTMF fails again. There is not even a re-register necessary for DTMF to fail.
Codecs set at Voxalot for Budgetphone: g729;g723;gsm;g726;alaw;ulaw
Codecs set at Nokia E71 for Voxalot: AMR;iLBC;G729;CN;PCMU;PCMA
Update: VoIP, I still don't get it. After changing codecs set at Nokia E71 for Voxalot to: AMR;iLBC;G729;CN && setting Voice activ.detection on for codecs 1-3, now it doesn't matter whether "Optimize Audio Path" is set to "No" or "Yes", both sent the DTMF digits correctly.