Roaming issues - being assigned temporary local mobile numbers

Impetus

I was trying to reach a friend recently on one of his mobile phones. He has one on an Irish network and one on a network based in the country where he lives.


He was roaming, in the Caribbean on a yacht. I did not know he was sailing at the time, or I would have used the yacht’s satellite internet to email him or satellite phone connection to reach him.

I called his main home mobile number (which is based in a European country), and spoke in French with a guy in Guadeloupe – who had no knowledge of my friend – even though ‘he was using my friend’s phone number’ at the time. I tried it several times using several networks at my end, and kept reaching the same French speaking guy.

Next I tried his Irish mobile number, and spoke in French to a lady in Guadeloupe who also did not know my friend. Both were complete innocent strangers.

To cut a long story short, when one roams, one is assigned a local number belonging to the roamed-in network. The network should check with the phone every 20 minutes or so, to ensure it is in coverage with the phone or vv.
If one is travelling by aircraft, one puts the phone into either flight mode or power off. This sends a SS7 signal to your home network that you have gone offline. If you have divert to voicemail on un-reachable for example in place under this condition, your callers will go to voicemail.

If you are on a yacht, or cursing on a ship, your phone will slowly move out of coverage as it heads for sea. Some networks have limited numbering space assigned to their VLR (visitor location register database). I suspect carriers with limited numbering space are recycling visitor temporary number assignments very quickly. One can imagine this in the Caribbean at this time of year with all the tourists and yachts visiting the region.
So, I suspect when his phones on the boat stopped responding to ping signals from the networks, the networks they were roaming on quickly re-assigned the Guadeloupe phone numbers that both his home networks had been assigned to divert calls to other people. Hence me ending up speaking to these French speaking strangers.

I don’t know if he was roaming on Denis O’Brien’s Digicel or Orange (as Guadeloupe is in France). Whoever it was needs to ensure that before they re-assign a temporary roaming number on their VLR that they send an SS7 message to the subscribers’ home network advising them to stop diverting calls to this temporary number in +590 country code.

From a phone users point of view, if you are sailing or on a ferry or ship, it is probably best to put your phone into flight mode while you still have coverage on the network you are about to leave. By all means switch the phone on again when you are more than say 40 km out at sea from the old network, so that you get coverage from the destination country’s network the moment you come within range. His destination in this case was Antigua which does not have a country code, and is part of the +1 North American Numbering Plan.

It was easy to tell which country was providing the call connection. In Guadeloupe, the ringing tone was generated in my mobile phone, as is the norm in Europe. In Antigua, the ringing tone was American sounding, sent down the audio line by some mobile phone switch in Antigua – wasting bandwidth while I was waiting for somebody to answer. Ireland has the same problem – when you call another Irish phone number the ringing tone uses an audio channel to convey the British sounding ringing signal, because Irish networks do not allow the mobile phone generate the ringing tone. If the mobile phone is allowed to generate the ringing tone, it is a greener solution because countries like Ireland and Britain have wireless spectrum capacity and cellsites wasted carrying the British ringing tone to the calling phones, while the callers are waiting for them to answer. Occupying probably thousands of voice channels every minute.