The traditional port for SIP messages is 5060, but we recommend you use port 5080 whenever possible.
The simple reason is that by running on port 5080, you hide the traffic from routers and firewalls that might try to "help" you by modifying the SIP signaling but often just end up breaking things. For a more detailed explanation, review our discussion of SIP ALG and how it can get in the way. Where possible, the communication should be between port 5080 on your device on port 5080 on our servers. Some ALGs will still recognize SIP traffic even if just one side of the conversation is using port 5060.