Depending on your configuration, TCP or UDP port 5060, and UDP ports 16834 to 32768. We recommend to use TCP or TLS for SIP signalling.  When we say open, we really just mean 'not filtered', as Standard NAT configuration will allow the inbound stream through those ports, once the outbound stream has established the route.

Additionally, you should generally ensure that any 'ALG SIP Helper' is disabled on your network, as these tend to be unhelpful in reality.

The sip.audio website's SIP Usernames configuration page has a setting for Relay mode.  'Smart' is the default setting, and is the safest option for navigating through NAT, but it only works with our preferred codecs (Opus, G722 & G711).