The likelihood is that they bock certain port numbers - or to be more
technically correct only open certain ports (there is a difference.)
Having said that, apart from the port numbers I would have thought that
VoIP is just data so unless they are deep examining it I don't see why
it shouldn't work. VoIP certainly works over my mi-fi when we are away
whether I use 3 or EE. Having said that we tend to use WhatsApp for
speech calls as it is more reliable and that works on 3, VF, EE, and any
overseas SP as well.
One way to find out is to use a mail client or two with different port
numbers and see if your mail gets through. In France there is often free
wi-fi around the Mairie's (mayor's) office which is on Orange France
(what used to be called Itineris in its nationalised days.) However just
about the only thing you can do is surf the web which uses port 80. Try
a mail client which can use any of a range of ports depending on whether
it is encrypted or not and your connection won't work. You cannot use
VoIP over it either. However go to the wi-fi in and around the Tourist
Information office and 99% of the time it will be fully open.
It may be the same with EE over here because if you are using VoIP they
are - in theory at least - loosing revenue.