I am looking for a Windows SIP client/softphone that is capable of saving notes per number and bringing up saved notes whenever said number calls again.
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My tSIP has option to save text with contact and bring it on incoming and/or outgoing call or manually if you configure "Contact note" button. Local and global hotkeys can be assigned to buttons, so it might be semi-convenient. Personally I would prefer opening note on request than automatically.
It is very, very basic though - just text in ugly window. If you have any kind of CRM software or dedicated software for taking notes that can be interfaced with e.g. command line, opening page in a browser or sending HTTP request it probably could be way more convenient and powerful. tSIP has a dozen of scripting events and configurable buttons to which scripts can be assigned, using e.g. ShellExecute() function.