Similar problem here, any progress on it?
From what i see the actual problem is not in OpenVPN providing default route (in my case there are only some static ones), but in the fact that for some reason Zoiper seems to completely ignore nameservers that were added by OpenVPN using "dhcp-option DNS x.x.x.x"' lines. Tried with both "push"-ing them from the server and setting them locally in the config. In both cases Zoiper seems to be the only app that fails to use OpenVPN provided nameservers.
I suppose in your case it also failed to use the nameserver, provided by OpenVPN. As for the nameserver from your mobile provider it did not work because when routing everything via OpenVPN your source IP is no longer one of their own and their nameserver is most likely set to ignore queries from foreign IP's.