My iPhone 5s keeps muting and dropping calls regardless of location

Over the last few weeks I've noticed my iPhone 5s has started to randomly drop calls ("Call failed"), regardless of whether it's on the same network as mine, or whether it's an iPhone or not. This happens in all locations, it's not specific.It also mutes calls multiple times during the conversation - as though the other person had put me on mute. There is no predictable length of time into the call, no predictable period after which it then resets, and it doesn't matter whether it's to an Android or iOS device, nor which network it's on. It also happens whether I'm holding the phone, on hands free, or on speaker. Over the last 2 days it's now developed the very frustrating habit of refusing to send texts: it gets to 95% of the green bar, then says it failed. Today it took me multiple attempts and even several reboots of the phone to get one to go. This seems to happen whether I'm on 4G, 3G, no cellular, wifi...

I called Vodafone today who have run all the diagnostics and confirmed there is no problem with my account or settings. I'm at my wits end! I'm wondering if it's to do with the iOS8.4 update - in which case can I roll back somehow?