Dual SIM call uses wrong default line

Question

djsomen OP

Created 1w

Replies 3

Boosts 0

Participants 3

Hello,

I am investigating call behaviour on a dual SIM iPhone.

Observed behavior:

A specific line is selected in Settings > Cellular > Default Voice Line.
Our app initiates the native call flow by opening a phone-number URL scheme with the destination number.
The native Phone app does not always place the call using the selected default line.

Expected behaviour:

The native Phone app should place the call using the line selected in Settings > Cellular > Default Voice Line

Questions:

In this flow, is line selection fully controlled by iOS?
Is there any supported way for an app to ensure the native Phone app respects the selected default line?
Is this a known limitation or known issue on dual SIM devices?

Any clarification on expected iOS behaviour would be appreciated.

Answered by DTS Engineer in 905628022

Thanks for the clarification.

is cellular line selection entirely controlled by iOS

Yes.

And IMO it should always default to the user’s chosen line. If you’re seeing different behaviour then I encourage that you file a bug about that.

Please post your bug number, just for the record.

ps It’s better to reply as a reply, rather than in the comments; see Quinn’s Top Ten DevForums Tips for this and other titbits.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Answer 1

DTS Engineer OP

Apple

1w

You added the CallKit tag to your thread but reading your post it doesn’t look like you’re using CallKit. Rather, you’re opening the default phone app via a tel URL. Is that right?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Answer 2

DTS Engineer OP

Apple

1w

Recommended

Thanks for the clarification.

is cellular line selection entirely controlled by iOS

Yes.

And IMO it should always default to the user’s chosen line. If you’re seeing different behaviour then I encourage that you file a bug about that.

Please post your bug number, just for the record.

ps It’s better to reply as a reply, rather than in the comments; see Quinn’s Top Ten DevForums Tips for this and other titbits.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Answer 3

DTS Engineer OP

Apple

1w

...always default to the user’s chosen line.

One qualifier on the "always" here is that I suspect there are edge cases where we do fall back on the secondary SIM. I haven't specifically checked each of these, but, for example:

If the secondary SIM is the only one that has a working connection.
It's possible/likely that there are other heuristics at work that to try and do the "right thing", which isn't necessarily going to be using the default under all circumstances.

That leads to here:

The native Phone app does not always place the call using the selected default line.

Can you provide any more specifics about the scenarios where this is actually happening? This is one of those situations where the standard behavior is straightforward, but becomes complicated once all the edge cases start coming into play.

__
Kevin Elliott
DTS Engineer, CoreOS/Hardware