Dual-SIM question (use SIM 1 for talk&text, and SIM 2 for data, simultaneously?)

[MassDeduction]

It is my understanding that the dual-SIM Windows Phones are also dual-standy, meaning both accounts aren't active simultaneously... or are they?

Microsoft's FAQ on the subject suggests you can independently select which SIM is responsible for voice, SMS, and data. If you assign talk&text to one, and data to the other, will that combination work simultaneously? The tech document makes it sounds like it will. Has anyone tried it? When my contract up, I'm giving considering to a cheap talk&text option for one SIM, and an inexpensive flex data SIM for the other SIM tray. I think that may suit my needs best (I'm in Canada). But first I need to know if it's technically possible. I'd be hoping to do it on a 640 XL, if that makes any difference.

 

[RumoredNow]

My understanding is that the Dual SIM in Windows Phone is considered "Active" meaning each SIM may be in use at all times as opposed to "Standby" which means SIMs have to be actively switched between.

I have a dual SIM Blu Win HD. It's my first ever Dual SIM that I have owned and I've had it about a week. I've poked all the settings pretty thoroughly. I only have the one SIM right now. However, it looks completely allowable from the settings and matches everything I have read that both SIMs will function at the same time.

 

[MassDeduction]

If it's dual-active, why do you have to choose which is the active SIM for different functions (voice, SMS, and data)?

 

[RumoredNow]

If it's dual-active, why do you have to choose which is the active SIM for different functions (voice, SMS, and data)?

You are dictating which functions are used by which SIM. Training the slots to know what to expect.

For instance:
SIM 1 - turn use data off - pin Phone 1 and Messaging 1 to Start
SIM 2 - turn use data on, configure APN and MMS - pin Messaging 2 to Start only if you wish to send MMS

 

[MassDeduction]

I can confirm that the Blu Win HD, and every dual-SIM Lumia I've looked at (including the dual-SIM variant of the Lumia 640 XL LTE, the highest-end dual-SIM Lumia yet), are dual-standby. I've been following a thread about it on HowardForums for someone who bought the Blu Win HD and couldn't get it to work, and it says "dual-standy" right in the product listings on Microsoft's website for each of the dual-SIM Lumias which leaves little doubt.

Dang... so unless the phone is capable of using two SIMs simultaneously for two different functions (such as one for voice and the other for data), then it's very doubtful that this is going to work for me, sadly. I wonder what would happen if you were using the mobile network for data on one SIM and someone called you? I guess it would go straight to voicemail?

 

[RumoredNow]

So it looks like you are correct that it is call active on only one SIM. But I think it will work for exactly what you want. Put your voice plan SIM in slot 2 (since it supports 2G only) and turn data off in that slot's settings. And your data plan in slot 1. Voice and SMS go through SIM 2, Data through SIM 1. The only problem is MMS where you have to switch to SIM 1, send mms, switch back to 2 to get SMS... Receiving MMS would be a pain as most would send it to your number in slot 2 (voice plan).

 

[RumoredNow]

I'll pull my wife's SIM later today and test the above suggested set up...

 

[MassDeduction]

Looking forward to your testing. I wonder if SIM slot 2 will truly be limited to 2G only, though? I know it is for the Blu Win HD, but it's a 3G handset so that's only one step down. I am hoping that the second slot is at least 3G on the Lumia 640 XL LTE, since it's an LTE handset and 2G would be *two* steps down.

I'll be checking this thread eagerly for your tests either way!

 

[RumoredNow]

I ran into a snag.

It didn't matter how many times I tried. I could not get my Win HD to see a card in slot 2. These are valid SIMS with service. It just would not take. I rebooted my phone about 12 times today switching 2 different SIMs between the trays. My wife needed her SIM back. I'll try again later. I may have to return this phone. It was an "open box" item on Amazon. Not sure yet if the SIM 2 tray is bad. Have to definitely check further.

 

[MassDeduction]

Thanks for the testing. Looking forward to hearing more when opportunity present itself!

I wonder if your settings were part of what was at issue? If you have voice set to SIM1, maybe it can't do anything at all with SIM2. But you may have checked for that, this was speculative on my part.

 

[MassDeduction]

This explanation of how a dual-standby phone ought to work makes things pretty clear: https://www.gogi.in/difference-dual-active-dual-standby.html

So it seems to me that if you set voice to one SIM, and data to another SIM, since the phone has only one transceiver then if the phone is accessing the mobile data network that it would block out the phone from being able to receive calls. If you try to make a call the phone would likely be smart enough to swap the transceiver over to the voice SIM and end data access.

The problem comes from the fact that a Windows Phone is typically almost always using data. It's updating Live Tiles, it's checking for new email, it's checking for OS updates, etc. Incoming calls would all too frequently be going straight to voicemail.

However, when you're in WiFi it would probably work fine as it would use WiFi for all data, and keep the transceiver locked onto the voice SIM.

Dang. I was really hoping I could have one SIM on flex data, and another on a cheap talk&text plan and have that combination work, but few smartphones (and no Windows Phones) are dual-active. A shame.

 

[MassDeduction]

This answer appears to be the final arbiter (and even includes a specific example that addresses my exact use case):

internet - What is the difference between dual sim active and dual sim stand-by? - Android Enthusiasts Stack Exchange

"There are actually three types of dual SIM phones.
•Dual SIM Standby (DSS): Only one SIM can be used at a time, 1 radio chip.
•Dual SIM Dual Standby (DSDS): Two SIMs can be used at a time, but if one opens a call or data connection, the other is offline.
•Dual SIM Active (or sometimes Dual SIM Dual Active) (DSA/DSDA): Two SIMs can be used at a time, 2 radio chips. This enables both SIMs to open calls and/or data connection."

All dual-SIM Windows Phones I've researched are the second-type, dual-standby. That means the phone can *listen* for incoming activity on both SIMs simultaneously, but once one is active then the other is offline. That's alright for voice if you're comfortable with talking on one meaning calls to the other go to voicemail. It's not so good for data, as persistent data use would mean all calls on the other SIM would go to voicemail

Wikipedia notes that a dual-standby device "allows both SIMs to be accessed through time multiplexing." I wonder if it's sophisticated enough to poll for incoming calls and SMSs on SIM-1 between times it polls for new email, etc., on SIM-2? I wonder how long the radio is really tied up in that context?

Everything I'm reading is strongly suggesting that when you're in WiFi, that you can make and receive calls with no disruption as the phone rarely (if ever) accesses the mobile data network when in WiFi, and it's for tiny snippets of time when it does happen. And I'm on WiFi at home, at work, and in many public places. So I could maybe live with that.