[Samsung Galaxy S7 Guides, News, & Discussion] Cynagen:
[size=+1]This guide was written as a primer to attaching RCS/IMS services to carrier networks that normally do not carry this type of traffic. Additionally, this guide will help you maximize your phone's capabilities by allowing you internet access while on a call simultaneously on networks other than LTE/WiFi. I expect in smaller towns where LTE coverage isn't as great this will help give you back that functionality. I will only be providing instructions for HSPA/HSPA+ (aka HSPA-DC or Dual Channel) as it's the only one I've tested, but since you'll have access to everything, you can experiment a little.[/size]

Prerequisites:

Rooted Samsung S7 (I used SM-G930F, may work with S6/S8, haven't tested), w/XAA CSC (TMB CSC may work as well, haven't tested)

Phone INFO

A little bit of balls, and willingness to deal with possible blowback by T-Mobile (they haven't said anything to me, yet, but this is breaking the standards!)

Steps:

Launch Phone INFO, open the main menu on the top left (3-bars), select "Secret Codes". (If you get any errors, hit YES to continue anyways, let it sit for a minute and if it doesn't stop searching, just tap to stop the searching early.)

Scroll through the secret codes, look for the app "Settings" with applet code "com.samsung.avdp.imssettings.SecretCodeReceiver" (On my SuperStock 2.8 S7 it's code number 467), select it and finally select Launch 0.

In the IMS SETTINGS display you now have click on IMS Profile.

Select TMobile LTE/WiFi

Select Network Enabled (>)

Here you can select which network types you want to enable IMS service on, the options "lte" and "wifi" should already be checked. You can now check "hspa" here as well for this guide.

[highlight]NOTE:[/highlight]

If "lte" is true 4G LTE (All carriers);

"hspa" is T-Mobile's original "4G" (high speeds >10mbps, slightly higher latency, rolled out to compete with Verizon's speeds prior to LTE)

"umts" is "3G"

"gsm" is "2G"

"ehrpd" is Sprint/Verizon's "EVDO" (for Sprint/Verizon users this may be an option instead of hspa?)

"tdscdma" is Sprint/Verizon's "3G".

Press back on the phone, next click on Service (>)

Scroll down to the "HSPA" section, you'll see all the services are blank, we need to enable which services to use on this type of network;

mmtel

smsip

slm

im

ft

presence

euc

Press back on the phone, and finally hit the Save (disk icon) button at the top, and back out completely.

Now when you're on HSPA/HSPA+ on T-Mobile's networks, you'll have the VoLTE icon and calls will be handled over data versus tying up the entire radio. (Check out my screenshot!) You can be bold enough to enable additional networks, though I caution that none of the other T-Mobile valid options umts/gsm will likely have enough throughput or low enough latency to be effective at all. Additionally, I noted when a call started, the phone took a little bit to adjust the jitter buffer to play the call smoothly, resulting in my voice coming through the other person's end garbled and jittery for the first few seconds (they came through fine to me). Once the call is up for 5+ seconds it's likely fully adjusted and you should have no problems afterwards. Do remember that because of the additional latency on the lower networks, you are going to have more delay in your conversation, though on my test call in the middle of the California desert along the I-5 between SF and LA, it wasn't noticeable on either side of the call. During that test call, I kept my Google Maps updated with the latest traffic conditions on my car's tablet via hotspot, so mission accomplished (for me at least).

Let me know in the replies if you attempted this tweak, where you tested, and if it worked for you!