When you use fring the E65 talks to the fring server using their own proprietary voice protocol. When fring talks to voxalot, standard codecs are used which explains why it is working.
According to the
Nokia Wiki the supported codecs are G.711, iLBC, G729
Given you can can *600, *500 and another voxalot number (my IVR) I would be looking at your parents side.
Just out of curiosity, are you able to reach your parent voxalot number via
SIPBroker - PSTN Numbers