My network keeps dissappearing

I have no network on my phone and there is not an issue with voda as i dont get network in or out Ive removed the sim and replaced it but network not working so unable to make / recive calls this is the second device this has happened on they are refurbished and only happen after a ciuple of months use this current one I cant even get it to go on to wifi calling despite the other phones I had allowed this feature so am at a loss thank you