Seeking a "softphone" app for iOS I'm moving towards IP-telephony and need a SIP-client for my mobile phone.

My first choice would've been Linphone, because that's what I use on my desktop, but it disconnects from the provider every time the phone goes into "standby"... I can make calls using it whenever I want, but, if somebody calls me on the VoIP-number, when my phone's screen is dark, they just get a message stating: "The subscriber is not available".

According to my VoIP-provider, this is a known problem -- I wonder, if there is a SIP-client, that solved it... Obviously, Skype and WhatsApp don't suffer from it...

If it matters, the VoIP provider only accepts UDP (sensible) traffic, and ZRTP is the sole means of encryption.

The family's iPhones are 5c, 5s, and 7 -- with the latest iOS at the moment.
I'm using Acrobits Softphone, and it is working very fine for me.

It supports many VoIP providers - here is a list: https://www.acrobits.net/about-us/partners#tab_verified-voip-operators.

ZRTP is supported too: https://doc.acrobits.net/general/secure_communication.html