My friend setup a zoiper account, with a UK number. He has been trying to call me without success. Do I also need the app installed on my phone? We both use Android's.
Not at all, I don't have it but we use it at work and my co-workers are able to call me without any issue. It might be a mistake in his configuration/setup or issues with is SIP (maybe, I'm not a pro)
No, you do not need Zoiper on your phone. Zoiper to Zoiper calls are free, but he can use his Zoiper to call your number. It sounds like a problem between his configuration and his VoIP provider.