Step 1, is to change your default SIP port from 5060 if the handset is still set to this. You can log into the phone manually to change this, and use something like 5065, 5066, 5067.
Alternatively, you can speak your VoIP provider, but hopefully this will already be set to an alternative anyway.
If you are using a Yealink handset, you may need to make some further changes.
If you follow the steps below, this should resolve your problem.
Log into the phone interface via your browser. This can my found from your handsets on screen network settings.
1. Disable IP Call feature:
IP Call feature allow the SIP phone to make and receive calls by IP address
(Without SIP account)
1.1 Change by WEB UI path: Features->General Information-> Allow IP Call
（A yealink T32G/T38G model cannot support configure it through the web UI）
1.2 Change by Auto provisioning parameter:
# 0-disable; 1-enable; default value is 1 (Enable)
2. Enable SIP trust feature:
SIP Trust feature allow the phone to only accept the message from the trusted server for specific account.
2.1 Change by auto provisioning parameter:
# 0-disable; 1-enable; default value is 0 (Disable)
2.2 Change by web UI path: (only V73 or higher firmware support. for W52p/W56p the firmware should be V80 or above, if the W52p firmware is V73 or below, should change the setting via auto provision): Accept Features->General Information-> SIP Trust Server Only
Each phone manufacturer will vary for a fix to the problem, but the best starting point is to ensure you always have the latest firmware installed for you phone.
Jibba Jabba come across problems like this every week and implement resolutions to help both new and old clients. We always strive to push out the latest firmware updates to existing clients as they become available too.