Third party VOIP providers

For my work, I have to use a Greek VOIP from the website www.omnivoice.eu. Their servers are:

sip.omnivoice.eu, 

sip1.omnivoice.eu,

sip2.omnivoice.eu, 

sip3.omnivoice.eu, 

sip4.omnivoice.eu,

sip5.omnivoice.eu, 

sip6.omnivoice.eu 

When I tried to connect to them a few months when I was in the USA, I could not. 

Are you blicking third-party VOIP providers? Are these ports blocked?

I have a Samsung A32 5 G which I got from T-Mobile when I signed up.

Would it be possible to open these servers up either sip.omnivoice.eu or sip6.omnivoice.eu?