Short answer: no.
In order to always be able to connect, at least one of the parties has to use a Chatmail server, because Chatmail servers (will) come with a TURN server. Without a TURN server it’s not guaranteed that the two parties can connect to each other. Sometimes they still can, (by using just a STUN server, which is used even for non-Chatmail accounts), but in most cases they can not.
AFAIK this is still under consideration, i.e. whether to just disable the “call” feature for non-Chatmail accounts, or whether to make them use some concrete third-party TURN server (such as the default Chatmail relay one).