Every few years, it’s interesting to do a reality check of mobile device or network features considered as stable or perhaps even legacy. And that’s because every now and then, things do change while you are not looking, and sometimes even for the better! Case in point: Call forwarding settings for voice calls while roaming.
When I last checked this many years ago, VoLTE roaming was still in its infancy. When roaming abroad on an LTE network, voice calls would make the mobile device fall back to GSM or UMTS for the voice call. That’s called Circuit Switched Fallback (CSFB). The problem back then: Some network operators blocked the change of call forwarding settings via USSD short code from abroad due to the fraud potential and the security limitations of the SS7 protocol. In other words: When you were abroad, you could not change those settings anymore. Pity.
Fast forward to today and VoLTE roaming has become the norm rather than the exception. When VoLTE is active for a subscriber, call forwarding settings are not changed by falling back to GSM or UMTS, but by using the XCAP protocol over IP, often by using the Home Operator Service (HOS) APN default bearer. This is much more secure because the subscription is authenticated during the process. So would this also work abroad? When I was recently in Hungary, I gave it a try in a network that had a VoLTE roaming agreement with my home network operator. And indeed, when I interrogated or changed, my call forwarding settings, the HOS default bearer was opened, an XCAP HTTP GET/PUT request was sent, and the settings were changed as requested. Nice, problem solved!
But wait, what about roaming in a network without a VoLTE roaming agreement? Before trying, I had assumed that CSFB fallback would be used and probably rejected. But no! Much to my surprise, the device I tested this with also made an XCAP request of the HOS APN dedicated bearer, and I could interrogate and modify my call forwarding settings despite being on a network without VoLTE roaming support. How interesting, I did not expect that.