Today I encountered a strange problem when I tried making a phone call using my home VOIP telephone. I could dial out, but there was no sound coming through, which is pretty confusing. At first I thought it had something to do with the firmware update I did recently on my Siemens A580 IP. After spending half an hour troubleshooting it appeared to be my VOIP provider uses another sip server to talk back to my Asterisk server. After adding this new server ip to my firewall configuration the sound is coming through again, which is the essence of telephony.
Added firewall rule:
access-list 101 permit udp host 126.96.36.199 host <my asterisk server ip> access-list 101 permit udp host 188.8.131.52 host <my asterisk server ip>
I think this could be useful for other Budgetphone VOIP users experiencing the same.