Sorry for late reply.
We were busy doing other stuff.
We changed ports (now something like 44333) and we are achieving better results - not the perfect solution (we still need to set up an extra configuration and set it up as a default configuration setting) but now
we are able to use the voip config settings to which we supplied the prov file and it works. Still, the user has to manually choose it in "settings > configuration > default configuration settings" but now at least customer wil not need to manually create a personal config;
now when user switch from wifi to mobile data back and forth, there are no annoying messages popping up.
NOTE: the preferred access point is still the operators. It looks like it is enough to have the default configuration settings set to something different than the operator account.
This means that, so far, we are not able to make this set up in a way that user doesn't need to change settings manually but the complexity for him has decreased. We would like to pursue trying to find a solution where we would not need to change this setting manually but we are not optimist about it. So I guess we will need to settle for this, for the moment.
Thank you for your help.
About your question: We are from Portugal and here the dataplans are a bit expensive so, we rather use wifi. Our middlet (we are still building it) works as a support to our voip services (account status info, dialed and dialing numbers, etc).
I'll post here if we move forward in our case (if we are able to completely avoid the need for user to manully change settings).