Incoming voice call, auto-answered:
So it’s not documented as a response to AT+CHLD
Clearly, there is network service - because a call is up!
So what does this really mean?
I get the same on pre-pay SIMs from Vodafone, Orange, and O2 (haven’t tried T-Mobile yet) - all UK.
I have now - the behaviour on T-Mobile is the same.
However, on Virgin (which is an MVNO using T-Mobile) the hold operation does work successfuly, and I can successfully dial a second call; but attempting to conference the 3 parties together (AT+CHLD=3) now fails
The problem doesn’t seem to be in the Sierra Wireless stuff - I get similar results if I try it on my phone.
I guess it’s because I’m using pre-pay SIMs, and the networks don’t provision this on pre-pay?
I still think Sierra Wireless is giving the wrong error message, though…
It means that this particular network service (Multiparty) is not available - it’s not saying that there is no service at all!
This should at least be more cleary documented, and preferable a distinct error code.
Yes - on the networks that support it at all, it’s only available with a contract.
All of the UK networks’ Customer Service (sic) lines have been utterly hopeless on this - continually giving advice that was unhelpful at best, or just plain wrong! 
The summary seems to be:
Vodafone: Supported on Contract only, needs a call to Customer Service to enable it;
Orange: Possibly supported on Contract only (unclear);
T-Mobile: Not supported at all;
O2: Not supported at all.