I recently got an asterisk phone system Zycoo UC50v2 and Cisco 502g phone. This is not the first time i have used the system. However for this particular install with cisco phones randomly Cisco phone will show service unavailable when we try to dial out. Sometimes it works but when it shows the service unavailable message it will stop working for hours or days. I have captured a sip debug trace which can be found out https://pastebin.com/kYmE3aq5
the users are on local lan and we only have single analogue line to ISP. Any help is appreciated.