[inquisitor]

All German providers (no matter if "real" MNO or MVNO) are forced by law to export numbers upon request. As number portability is subject of a very old EU-directive (97/33/EC) it should have been implemented to national law in all EU-countries meanwhile.

However German mobile providers are not obliged to import numbers. So you can always export an existing number, but not all operators will import such.

Fonic e.g. still doesn't support number importability, but solomo definitely does.

But be warned, that all operators will charge an amount of € 25-30 for the export, which needs to be topped up in advance. Thereafter you'll also need to send a letter of cancellation to the exporting operator ("Verzichtserklärung") before you instruct your new operator to import the number.

You should also know, that some operators do only import numbers if you tell them to do so while ordering their SIM (e.g. simyo), whereas others will import numbers even afterwards (e.g. solomo). So solomo subscribers can decide anytime to replace their existing number by importing another one, while simyo customers need to order a completely new SIM if they want to import a number. They can not replace their old number by an imported one.

Lastly I should mention, that when porting numbers the importing operator will send a request to the exporting operator with firstname, lastname and date of birth of the customer. If this information is not absolutely identical with the data of the exporting operator, they will refuse the request. So you need to provide absolutely the same details to the importing operator as the ones the exporting operator has from you. If you have abbreviated your name or if you have provided a second firstname, you'll need to do this with the new operator, too.
Again solomo are the best choice at this point, because they allow entering separate information for the number-import-request than the information provided upon ordering. So solomo customers can theoretically even import somebody else's number, since they can manually provide the data for the request.

I hope my explanations are kind of understandable.