Ok, i found the setting to keep Zoiper running, it was hidden under Settings -> Apps -> Zoiper -> Akku -> Screen off: stay running (I don't know how it's called in english)
I didn't expect it there and it took some time to find the setting again - not positioned very well.
But shouldn't this setting be set by default? I didn't set this for other apps that run in the background.
Now i have some additional questions:
Is there a way to amplify the microphone, i found one (or more) settings to amplify the speaker, but nothing for the microphone. All people called told me that i was not good to hear.
Are the codecs selected automatically by available bandwidth or only by availability of the sip provider. Or in other words, does it make sense to have multiple codecs selected or is it better to set a specific codec for every account
I think about to create one account for every type of connection (Wifi@home[HQ], Wifi@somewhere[HQ/VPN], Mobile[LQ/VPN]) all with the same sip-provider.
Are there any things i should consider, especially when using over mobile connection and/or through VPN - i had a really bad quality when used over a limited mobile connection (~50kBit).
Is the quality better if not used through VPN, in that case i have to modify my Fritz!Box, so i can set a non-default port, which is blocked by my mobile provider, but voip is not forbidden by their terms and conditions.