I've had an Xtrons unit in my B7 A4 for around 5-6 years and it's been great. All these Chinese Android units (Xtrons is just a branded distributor - the units are made by a host of Chinese companies) come with a generic canbus decoder for steering wheel controls, unlike brand HUs supplied controls which are much more specific, such as those from Enfig. Normally and this is the case with Xtrons the decoder you buy is already set up for your model but they will still be lacking in full control, Voice/telephone and pause for example. You have to dig in and use open source tools (some of them have dip switches too and you may have to change those for your car; you may also have to change Factory settings too for your model). I had to also use something called MTUTools, I think, and the voice button now works with Android Auto very well, to invoke Google Assistant, for calls, navigation, Spotify etc. In fact, I use voice control in the main and hardly ever touch the screen. BUT that is just Android Auto and I have to use a paid app called Headunit Reloaded to both fire Android Auto and programme its codes through MTUTools. The app cost 5 quid and it works perfectly. I also use a Motorola MA1 AA dongle, hidden in the glove box for wireless - again works perfectly.
I don't know of a similar app for Carplay. There are dongles that come with their own app to do the same and they are said to work well but I am not sure about steering wheel controls. Newer Android units have both AA and Carplay built in, often now both wireless, but they work in the same way; effectively the 'dongle' is built into the unit. Unlike both Headunit Reloaded and the Motorola they are not officially sanctioned and are effectively cracked workarounds; though as I say some of them work well. For official Carplay you really have to look at Pioneer etc.
I've seen lots of people very unhappy with Xtrons but I have been very lucky with mine: it has great sound and has been utterly reliable; and since it has 4GB/32GB it's still fast and I have no need to upgrade (they are a lot faster now) especially since I'm only using AA (Sygic and Spotify etc. run great direct from the HU too though). Re the BT and calls, I agree that re-pairing might be in order. The in-built mics on all of these units are pretty p*** poor too and an external mic doesn't cut the internal one neither. You have to adapt and mod. I have my external mic now in the Audi OEM placement and I no longer have issues. They can work very well these units but they are not always plug and play; you have to fiddle with them. The accessories (reverse camera, TPMS etc.) are almost always rubbish though and usually crap out quickly and it is better to source quality branded ones from elsewhere. As I say the HU itself has been rock solid though, for me. Hope this helps.
