The "SIP 400" error comes from your VoIP server for some reason, and usually means "Bad request".
This error might appears if there is a problem with the number formatting, e.g. empty spaces or bad prefix - "+" instead of "00" or vice-versa and etc.
However this error messages comes from the server itself and only the server administrator can tell why exactly it is appearing.
If you receive such error messages, we recommend that you contact the server administrator/ VoIP service provider for more detailed information and assistance.