Maybe this problem has been sorted by quicker brains than mine.
Here by accident I found how to add to Android Zoiper contact list.
1. isolate the device from internet( wi fi and data)
2. Open Zoiper and dial the number you wish to add.
Connection fails.
3. Open Zoiper Call Log.
$. Click on the number you just tried to call and then Edit as per the dialogue box.
PS if you do not isolate the device from Internet Zoiper will try to connect the call.
Sorry if this is boring old news.
I am convert to Zoiper.
Cheers
PTF