Have you had the following error when you try to swap calls or put a call on hold. This article will bring together all of the causes and solutions.

Error message
Can't switch calls.

Background

Before we get started I need to explain a few things

This is a message I got from my provider which helped me understand things:

4G calling uses your 4G data connection to carry your phone and video calls, rather than the 2G or 3G networks.

It also works alongside WiFi calling, so you'll automatically switch between them without your connection dropping.

If your device supports 4G calling, you've already got access to it, but you may need to switch it on your phone's settings, or you can check in our device guides - choose your device, then tap Specifications and scroll down to Communication > VoLTE – 4G calling. It will state here if your device is compatible or not.

The following points are important:

• 2G/3G use a different technology than 4G/LTE for handling voices calls and therefore also Call Waiting is handled by the corresponding technology/protocol.
• Networks can enable/disable Call Waiting for both of these technologies independently

Prerequsites

• Update all of your apps. This probably wont make any difference but it wont harm.

Causes

• Call Waiting is not enabled on your contract
• Call Waiting is not enabled on your phone
• Call forwarding is redirecting all calls before you get them
• Carrier does not support Call waiting on their 2G/3G networks, only 4G/LTE and upwards.
• Phone/ROM not support VoLTE
  
  • Why is this specifically and issue. Most phones support Call waiting on 2G/3G networks but these networks are now being shutdown, but your phone is still working you ask! Carriers will start by shutting down all non-essentials services such as Call waiting on these old networks and hope that customers migrate naturally to newer phones that support 4G/LTE and of course VoLTE. so while the 2G/3G networks are still working these extra features are not. If you do not have VoLTE you cannot utilise Call waiting on the newer network.
  • When the 2G/3G networks are shutdown, if your phone does not support VoLTE then you will not be able to makes voice calls.
  • Modem is too old
  • Technology that handles calls changes, but this technology is not supported in older modems
  • Modern phones and carriers require VoLTE
  • The Custom ROM such as LineageOS, does not always support VoLTE
  • LineageOS that Support Volte? - F-Droid Forum - Is there any way to tell if a LineageOS phone supports Volte?
    • Notably anything older than 2016, does not support VoLTE, devices then used non-standard IMS stacks.
    • You can check the sources to see if it supports it, but even then your carrier may not allow it or it may lack the required configs for your carrier.
    • What exactly should I be looking for when I look at the source code?

      Check for an ims.apk in the proprietary.txt files for your device.
      And because some devices use a common device tree, find the command device tree
      name in the lineage.dependencies file and check the proprietary.txt files in
      that repo too.

  • Another way to see if you phone can use VoLTE is when you make a voice call, 4G/LTE changes to 2G/3G/H/H+
    
    • This is caused by the phone have to downgrade the connection type to allow a voice call to be made
    • VoLTE Questions And Where To Go From Here? : LineageOS
    • Users With VoLTE Working : LineageOS
  • Determining VoLTE support on a device : LineageOS
    • Due to the closure of 2G and 3G networks drawing closer all over the world - the latest date I'm aware of is mid-late 2022, and there are networks that will shut down earlier - devices must support VoLTE to connect to the network.
    • In order for VoLTE to work properly, carrier configuration and APN must be correc
    • For example stock Samsung OS supports it but LineageOS for S5 klte + variants does not support VoLTE (Voice over LTE).
    • You should be able to use the dialer code *#*#4636#*#* --> "Phone Information" to determine if VoLTE is working.
• Dialler storage and cache is corrupt
• Dialler does not have all required permissions
• Carrier configuration and APN are incorrect
  
  • Because VoLTE uses data, it is important for this to be working.

Solutions

These solutions might not work and might not all be relevant to your phone.

• Enable Call Waiting on your contract
  • Ring your provider and make sure this feature is enabled and available.
• Enable Call Waiting on your phone
  • or *#43# to check (add these notes
  • LineageOS 18.1: Dialler --> 3 Dots --> Settings --> Calls --> GSM call settings --> Additional settings --> Call waiting
• Disable Call forwarding
  • So you can rules this out. It can be put back on later.
  • LineageOS 18.1: Dialler --> 3 Dots --> Settings --> Calls --> GSM call settings --> Call forwarding
• Update Phone Modem (to give VoLTE support)
  
  • Stock ROM: Updating your stock ROM will also update the modem
  • LineageOS: You will need to update the modem manually by the relevant method (not covered here)
• Clear the Dialler storage and cache stores
  • I have seen people mention this one, but I suspect this only fixes the issue if it was working one day and then not the nexxt rather than after a phone or network update
  • Done through the 'App Manager'
• Open the Dialler App properties and make sure it has all permissions enabled
• Re-setup your APN
• Use WiFi calling
  • I have not tried this, but if you can install the app, just try and see.
• Get a new modern phone
  • This is a last case scenario

Other Notes

• The Android Dialler uses an API to communicate with the modem so it is unlikely to be the dialler causing any issue.
• What is VoLTE? - VoLTE stands for voice over LTE and it’s more or less exactly what it says on the tin. It's voice calls over a 4G LTE network, rather than the 2G or 3G connections.
• Call waiting commands (from Vodafone)
  
  • To activate - enter *43# followed by SEND
  • To accept the incoming call and end the original call - press 1 then SEND
  • To hold the original call and accept the incoming call - press 2 then SEND
  • To swap between calls - press 2 then SEND
  • To end present call and return to holding call - press 1 then SEND
  • To cancel - enter #43# then SEND
  • Phone Information - *#*#4636#*#* then SEND
• Vodafone 845 - Turn call waiting on or off | Vodafone UK - When call waiting is turned on, you can hear and answer a new incoming call while you're on the phone. Follow these instructions to turn call waiting on or off.
• How To Fix Error Message Can't Switch Calls Problem Solved in Android - In this blog, I am going to tell you How To Fix Error Message Can't Switch Calls Problem Solved in Android.
• How to Set Up Call Waiting on Android: 3 Methods | JoyOfAndroid