Newbie here!

Have Kamailio set up on a 512MB VPS. Created a couple of users, registered with Zoiper and can call between them.

I am looking for documentation to connect the server to other VoIP providers (Signalwire, Anveo, VOIP.ms etc.) to make & receive calls from the external network (DIDs) / PSTN  

Can someone point me in the right direction?

Thanks in advance.