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iPhones do not allow the application to run in the background, only Android phones do. Groundwire is an app that has push notifications and is kept alive by the Groundwire servers when on an iPhone. The app is a premium app.
My advice is to use Groundwire if you have an iPhone
Android allows Zoiper to run in the background, so you do not have to close the app. Just navigate away from the App to keep it running in the background.
They will end up fighting each other for registration. One of them will always be online, depending on which one registered last. Both will not work at the same time as calls are sent by our server to the last port that it registered from. Each time a device registers, it gets a different port
Analogue phones can be used if you purchase an ATA adaptor, and then install your Freshphone account on it. It plugs into an internet connection to register your account, and then converts the signal to Analogue and has a port that you can plug an analogue phone into. Grandstream make well priced models eg HT-701