I am certain large mobile operators like Vodafone uses voip to route calls worldwide. I also notice a similar problem when I try to make an international call out of the UK. The receiving party I am able to hear but they aren’t able to hear me. I am able to get the call working though, but only after 10 tries or so which cost me a few pence every time I try. I’ve informed vodafone several times about this and they still haven’t gotten it right.