The + plus sign is a placeholder for your local IDD Prefix (International Direct Dialing). For example it is 00 in most European countries, 011 in USA and Canada.
The international dial number for the Czech Republic is exactly 420 so for example you should dial exactly 00420381254870 in case your IDD is 00.
Please don't call to this number, fax to it directly.
If your Windows installation uses invalid international dial prefixes and you want to correct them please see these articles:
How to Change International Dialing Access Codes in Win3.x
How to Change International Dialing Access Codes in Win95