From FreeSWITCH Wiki

This page briefly outlines some typical Connection Refused errors as well as possible solutions

Contents 1 Type 3 1.1 Code 0 1.2 Code 1 1.3 Code 2 1.4 Code 3 2 External Reading

Type 3

Destination Unreachable

Code 0

Net Unreachable

Code 1

Host Unreachable

Code 2

Protocol Unreachable

Code 3

Port Unreachable 
Try correctly setting STUN on the device that's having problems (i.e. your IP phone)

External Reading

http://www.iana.org/assignments/icmp-parameters