Telecom - Staff Letter addressed to All Wireless Carriers

Ottawa, 12 April 2023

Our reference: 8620-C12-201312082


All Wireless Carriers

Subject: Request for information - VoLTE interoperability issues between networks and devices

It has come to Commission staff’s attention that some international roaming customers in the United States (US) are not able to make/receive voice calls due to Voice over LTE (VoLTE) interoperability issues between 4G and 5G VoLTE networks and devices. The purpose of this letter is to seek information on potential impact on Canadian networks.

This issue came to light recently in the US, after AT&T shut down its 2G and 3G services, and many European travellers to the US received text messages that due to network technology compatibility, traditional voice calls will not work, and for them to use data, SMS, and app-based calling.Footnote1

It appears that certain VoLTE-compatible mobile handsets lack interoperability with certain LTE mobile networks that resulted in the inability for users to make or receive voice calls. As the mobile wireless carriers had decommissioned their 2G and 3G networks that had previously acted as their fallback voice networks, the calls failed as VoLTE was the only available option to provide voice services.Footnote2 In addition, it appears that this VoLTE interoperability issue has the potential to not only affect international roaming users, but also Canadian roaming customers, as well as customers switching mobile service providers or mobile handsets in Canada.

The ISOFootnote3 /TC 204 Standards CommitteeFootnote4 WG 19Footnote5 has issued an information alert regarding this issue, the details of which can be found in the Attachment 1, “ISO TC204 WG19 Information Alert”. This information alert was brought to our attention by the Standards Council of CanadaFootnote6 in July 2022; and was also sent to Canadian wireless carriers.
The inability of subscribers to access voice services on 4G and 5G VoLTE networks, including access to 9-1-1 services and roaming, is a serious concern. As a result, Commission staff is requesting responses to the questions in the Attachment 2, on whether Canadian wireless carriers are experiencing the 4G or 5G VoLTE networks and devices interoperability issues as identified in the information alert, and if so, what measures are being taken by wireless carriers to prevent these interoperability issues occurring in Canada.

The responses to this Request for Information must be submitted to the Commission by 12 May 2023.

Please note that all responses to this request for information and any subsequent correspondence will be added to a public record and may be considered in future proceedings where the information is relevant.

As set out in section 39 of the Telecommunications ActFootnote7and in Broadcasting and Telecom Information Bulletin CRTC 2010-961Footnote8, Procedures for filing confidential information and requesting its disclosure in Commission proceedings, persons may designate certain information as confidential. A person designating information as confidential must provide a detailed explanation on why the designated information is confidential and why its disclosure would not be in the public interest, including why the specific direct harm that would be likely to result from the disclosure would outweigh the public interest in disclosure. Furthermore, a person designating information as confidential must either file an abridged version of the document omitting only the information designated as confidential or provide reasons why an abridged version cannot be filed.

Yours sincerely,

Original signed by

Noah Moser
Director, Broadband and Network Engineering

Attach. (2)

  1. ISO TC204 WG19 Information Alert
  2. Request for Information (RFI) Questions

cc:     Michel Murray, CRTC,
Noah Moser, CRTC, 
Vish Iyer, CRTC,


ISO TC204 WG19 Information Alert

Critical failings on systems using 4G and 5G ISO TC 204 WG19 and CEN TC278Footnote9 WG15Footnote10 have recently been made aware of some very serious flaws in technical interoperability over 4G and 5G, if you are operating an ITS system using 4G or 5G we draw these problems to your attention.

The problems were exposed when emergency and roaming calls in USA started failing after some operators switched off their 2G and 3G networks. And while this affects emergency calls and roaming, the problems might have far more reaching effects. It turns out that Voice over LTE (VoLTE) has been regularly failing, in which case the 3GPP standards require default to a previous generation. A failed 5G call will default to attempting a 4G call, and a failed 4G call will default to being a 3G call etc. So the failings of VoLTE / IMS remained hidden until the network provider turned off 2G and 3G, The reason for the failure turns out to be that there are thousands of options built into the 3GPP specifications, such that the sender needs to know the particular options used by both the receiver and the communicating equipment, and all parties need to support the same options.

The industry has not narrowed this down to a very few operating profiles (as they did with 2G and 3G). In a test/trials situations all parties are usually known and in dialogue with each other, so the problems do not show. But in real world situations the sender does not know the options supported by the receiver, nor the communicating equipment. Roaming therefore poses particular problems.

These problems are most serious for VoLTE and in particular emergency calls and roaming, but because the problem is that there are too many options in VoLTE & IMS, there is chance of failure in any 4G/5G system where the sender, receiving and communicating equipment are not intimate with each other.

Telcos and device manufacturers are aware of the problem, but commercial pressures seem to be working against their finding a solution.

We bring these problems to your attention offer the following advice:

1: Considerable care should be exercised when relying on test results. Insist on open tests involving several suppliers and networks to expose any challenges. Always include roaming in tests/trials.

2: Pressurize your authorities to mandate standardised communications profiles for critical services.


Request for Information

  1. Have you experienced VoLTE network and handset interoperability issues for roaming customers on your VoLTE network as described in the ISO TC204 WG19 Information Alert (“information alert”)?
    1. What is the nature of these issues?
    2. Have you conducted open tests, including roaming and emergency call tests/trials, involving several suppliers and networks to expose any of the identified challenges in your VoLTE network?
    3. What steps have you taken to resolve and address this VoLTE interoperability issue to ensure users on your VoLTE network are able to access all the VoLTE network services, including completing calls made to 9-1-1, no matter if they are your customer, roaming on your network, or the make/model of the VoLTE-capable device they use?
    4. If this interoperability has not been resolved in your LTE network, what is your plan to resolve it including all milestones and timelines?
  2. Have roaming customers from another Canadian mobile wireless carrier been unable to access your VoLTE network due to this VoLTE network and/or handset interoperability?
  3. Have you encountered this VoLTE network and handset interoperability issue when a customer brought a mobile device previously used on another Canadian mobile wireless carrier’s network or used a mobile handset purchased internationally?
  4. Do you have any restrictions in your VoLTE network that might potentially prevent one of your customers bringing a mobile device previously used on another Canadian mobile wireless carrier’s network from accessing VoLTE in your network?
  5. Have you decommissioned your 2G and 3G networks? If so, when did you decommission each of them and did you experience any VoLTE network and handset interoperability issues as a result?
  6. Are you in the process of or do you plan on decommissioning your 2G and 3G networks entirely making VoLTE the only option to make/receive voice calls?
    1. If yes, when do you plan to decommission your 2G and 3G networks?
    2. Are you proactively conducting tests/trials before decommissioning your 2G and 3G networks to prevent VoLTE interoperability issues in your network?
    3. Are you in discussions with other carriers or vendors in Canada or internationally on how to prevent or resolve any potential technical issues in your 4G and 5G networks as a result of decommissioning 2G and 3G networks? What solutions have been developed or implemented?
  7. Are you participating in discussions to address VoLTE interoperability with device makers, network operators, standards organizations, or a combination of these?
    1. If discussions are taking place in an industry working group(s), describe the working group(s), whether other Canadian and US carriers are participating, and what relevant tasks are being conducted to resolve this VoLTE interoperability?
    2. Are you working with your network equipment vendor(s) or mobile handset manufacturers to prevent or resolve this VoLTE interoperability issue?
  8. As a wireless telecommunications carrier in Canada are you aware of any other information of potential relevance to the Commission pertaining to the issues described above, in relation to the decommissioning of 2G and 3G networks and VoLTE interoperability, notably with respect to ensuring access to emergency services for all Canadian wireless users? If yes, provide all relevant details and supporting information.
  9. Are there any steps the CRTC should take within its mandate to ensure that at minimum, voice services including reliable access to the 9-1-1 network are available to any wireless user across Canada, irrespective of the user’s home network or device (which may or may not support VoLTE)?
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