Lufthansa's website says the following with regards premium economy lounge access vouchers, which suggests you buy them at the airport rather than the call centre
The purchased voucher for lounge access is valid exclusively for Lufthansa Business Lounges in Germany and the USA and at Charles de Gaulle Airport in Paris, in combination with a valid Lufthansa boarding pass with LH ticket number. Vouchers are available at the Lufthansa ticket counters of the respective airports, and also at the lounge entrances at Frankfurt, Munich and Dusseldorf airports.
The current admission charge for our Business Lounges is 25 euros in Europe and 35 US dollars in the USA.
The admission charge for the Lufthansa Welcome Lounge in Frankfurt is 50 euros.