Using netcat or telnet it is possible to test whether a server can reach another SIP server (SIP trunks) or a SIP endpoint. Netcat is my preferred method:
nc -vz ip.ad.dr.es 5060
If the above command says "Connection refused", it does not automatically mean the port is closed. It may indicate that the SIP stack on the other end only supports UDP! In which case, you should also try:
nc -vuz ip.ad.dr.es 5060
If at this point you still get a "Connection refused", then indeed there may be a problem on the other end.
How about SIP TLS (5061)? As per this link:
openssl s_client -connect 83.136.32.159:5061 -no_ssl2 -bugs
