Reliability of prepaids for foreign calls such as Lebara, Ortel... what to take into account? [closed]

There are many prepaid operators in Germany, that offer SIM cards for cheap foreign calls: Lebara, Ortel, Mobilka etc.

But I've heard bad opinions about their reliability and the connection quality. I've used Lebara, and honestly, I've had a lot of issues when calling, especially between 20 and 22. I've got a message 'Netz belegt', which would suggest, that the recipient was talking, but the number wasn't used.

However, it's hard to tell on the basis of a few usages is it a rule, or I was simply unlucky. So, when buying SIM card in Germany, what should I take into account? Are there any comparisons of reliability and quality of connections, especially in heavy traffic hours?