See No incoming calls - #9 by ttquattroman .
(The thread is about a different FXO device, but the issues are the same).
tl;dr: If you don’t need the device to register (it’s on the same LAN, connected via VPN, or it has a static public IP address) there is no trouble. You can use chan_sip with no trouble. Otherwise, you need to make some manual changes to the pjsip configuration files.