No it can't be changed, but that's not the point.
If the slides are to the left then the service is not available, this may be because of either network or phone restrictions;
i.e. if both the network and the phone are capable of providing the service then the slide will be over to the right, if either the phone or the network are not capable then the slide will be to the left.
So, for example, we know that Wi-Fi calling is available on the BT network so therefore your HTC10 was not capable of it. whether this is because of it's software or hardware is unknown
Edited to include the missed out word
According to various websites I have seen, both now and when I bought the phone in 2018, the HTC 10 is WiFi Calling capable. So I guess that means it is hardware capable, but the software is the issue?
The phone currently has the stock HTC Android v8.0.0 installed at the moment, but I was thinking about flashing it with Android 10. Do you know if upgrading the Android version would resolve this?
I don't know.
All I can say (for what it's worth) is that I bought my Nokia phone from Amazon, it's on BT, running Android 10 and Wi-Fi calling works fine even though MyBT tells me it doesn't.
Thanks for that.
Both Wifi calling and VoLTE are marked as provisioned, with the slider over to the right. Video calling and EAB?Presence are greyed out. Network type is set to LTE/UMTS auto (PRL).
Still the same old error message when I try to make a call though.
Just a thought, did you you get the usual text messages with configuration settings when you powered the phone up for the first time after fitting the BT sim and did you restart it.
Either way it may be worth restarting it again
Sadly, the Microsoft Panacea, turning it off and turning it on again, had no effect.
Incidentally, have you come across this problem before, where the phone seems set up for wifi calling, but can't see a connection to wifi? It's as if the left hand doesn't know what the right hand is doing, but then that is so often the case these days.
Funny enough I had a similar problem when I first got my current 'phone.
Phone said Wi-Fi calling preferred but it never worked, I went through much the same process as yourself and got no joy, then from out of nowhere it just started working. I never did get to the bottom of it and can't give a logical explanation for it.
You could swear the damn things are alive.
BTW My 'phone says "Mobile preferred" under wifi calling. Is it possible to change it to Wifi preferred?
I suspect it's not so much a case as Mobile having been selected as the preferred connection but more like WI-Fi calling has not been set as the preferred method, if I'm right, you'll see what I mean.
If you tap where it says Mobile preferred I suspect it will take you to another screen with a slide switch to enable Wi-Fi calling as the preferred method , if this is greyed out then tap it and hopefully it will come alive and your problem will be sorted