[Ring] SIP registration problem

[vwf]

Hello,

I have a SIP registration problem. iptel.org works fine, ostel.co does
not register using similar information. ostel.co gives either a status
"Host unreachable" or "Invalid". My ostel.co account is valid: Jitsi
works just fine with ostel.co 

Ring version: 20161123.1.3120ba5~dfsg1-1 (on Debian Stable)

Information used for ostel.co:
hostname: ostel.co
username: myname (without @ostel.co)
password: ********** (tested)
proxy: <empty>
DTMF: RTP
(advanced)
local port: 7060 (5060 is already in use)
published address: same as local parameters
use stun: stun.ekiga.net
SDP session negotiation: <default values>
(security)
<nothing, all default>

What is going wrong? What can I do to test?

Thank you