Contacts saved in .cdb files (Symbian phones mainly) can be exported to a common CSV format for contact exchange purposes with Nokia Suite. Maybe only certain versions are capable of this conversion, but this is probably your best shot. Alternative would be to somehow sync your Symbian with Gmail or similar email client and export the contacts from there.
Convert Symbian OS contact database file to Comma Separated Value.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert cdb to csv on Windows
A set of applications used to manage older Nokia mobile phones
Convert from cdb file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.