How do sim4travel, maxroam etc. work technically?

torput · **How do sim4travel, maxroam etc. work technically? -** 14-04-2008, 11:51

What's the catch with all these "receive calls for free while roaming" SIM cards?

In other words, could someone please explain technically how they work. I know GSM techonlogy pretty well and there is something either fishy or very clever in these services. Somehow it utilizes SIM toolkit + GSM roaming number.

andy · 14-04-2008, 12:12

How is it financed, or how does the SIM work, or both?

Obviously there is some incoming roaming cost, but these operators have taken a view that for marketing reasons they can swallow this in return for the income from the outgoing calls, or that the received incoming termination fees also subsidise it to some extent.

As for the technical detail, they all use callback; when a call is set up by dialling a number as normal, the user sees a screen message to please wait. Then the phone maybe actually sends something like *1xx*00nn...nnn#, where the 1xx varies slightly on different providers, e.g 117 or 137, and the 00nnn after the second * is the desired destination number. After a few seconds this user's phone rings ...

Some phones, or one or two of the SIMs, don't support dialling the destination as an ordinary number like that, but the code can be sent as above

Or other systems may be actually sending an sms format to trigger the call, but either way it's usually invisible to the user

With some of these, the user's phone number doesn't actually pertain to the country of the underlying SIM card, with either a landline or mobile DID forwarded to it.

Someone else can probably add a bit more technical background, or at least the right jargon sets of initials

torput · 14-04-2008, 12:24

Thanks!

So the callback system probably calls the MSISDN of the subscription and not the GSM roaming number. And some users with specific calling patterns can cause losses to these companies.

PhotoJim · 14-04-2008, 16:04

My guess (and it is just a guess) is that the revenue to the provider for the inbound call exceeds the cost for the roaming for that call in some countries, and those are the countries where inbound calls are provided for free. For example (and again, hypothetical only) they might receive 10 eurocents for the inbound call and pay 7 or 8 eurocents for the roaming charge.

These providers would not risk losing money on inbound calls because certain users will find and exploit these opportunities (it's known as arbitrage), and that would make the whole business model vulnerable to collapse.

DRNewcomb · 14-04-2008, 23:43

They use a GSM function called USSD (Unstructured Supplementary Service Data) which allows the roamer to send a free signal back to his home switch. This signal says to the home switch, "Call me back and connect me to number X" This allows the service provider to control the call costs real-time and not wait for the roamed carrier to send the bill as with regular post-paid roaming. The ideal "free incoming" roaming SIM will be issued from a small country with lax regulation where the carrier can charge a high termination premium. Some of these are special numbers that are charged higher than a normal mobile but many international long-distance carriers just average the costs and charge one rate to all mobile numbers in a given country. The high premium will cover the rate to make an international call to the roamed country plus their wireless surchage, which is all the carriers usually pay. (The normal practice in international roaming is to rip the customer off to the maximum extent possible.)

	(#1)
torput (Offline) Junior Member Newbie Posts: 2 Join Date: 14 Apr 2008 Country:	How do sim4travel, maxroam etc. work technically? - 14-04-2008, 11:51 What's the catch with all these "receive calls for free while roaming" SIM cards? In other words, could someone please explain technically how they work. I know GSM techonlogy pretty well and there is something either fishy or very clever in these services. Somehow it utilizes SIM toolkit + GSM roaming number.

	(#2)
andy (Offline) Senior Member Prepaid Prophet Posts: 2,128 Join Date: 10 Dec 2004 Country:	14-04-2008, 12:12 How is it financed, or how does the SIM work, or both? Obviously there is some incoming roaming cost, but these operators have taken a view that for marketing reasons they can swallow this in return for the income from the outgoing calls, or that the received incoming termination fees also subsidise it to some extent. As for the technical detail, they all use callback; when a call is set up by dialling a number as normal, the user sees a screen message to please wait. Then the phone maybe actually sends something like 1xx00nn...nnn#, where the 1xx varies slightly on different providers, e.g 117 or 137, and the 00nnn after the second * is the desired destination number. After a few seconds this user's phone rings ... Some phones, or one or two of the SIMs, don't support dialling the destination as an ordinary number like that, but the code can be sent as above Or other systems may be actually sending an sms format to trigger the call, but either way it's usually invisible to the user With some of these, the user's phone number doesn't actually pertain to the country of the underlying SIM card, with either a landline or mobile DID forwarded to it. Someone else can probably add a bit more technical background, or at least the right jargon sets of initials Last edited by andy; 14-04-2008 at 12:25..

	(#3)
torput (Offline) Junior Member Newbie Posts: 2 Join Date: 14 Apr 2008 Country:	14-04-2008, 12:24 Thanks! So the callback system probably calls the MSISDN of the subscription and not the GSM roaming number. And some users with specific calling patterns can cause losses to these companies.

	(#4)
PhotoJim (Offline) Senior Member Prepaid Expert Posts: 389 Join Date: 10 Dec 2006 Location: Regina, SK, CA Country:	14-04-2008, 16:04 My guess (and it is just a guess) is that the revenue to the provider for the inbound call exceeds the cost for the roaming for that call in some countries, and those are the countries where inbound calls are provided for free. For example (and again, hypothetical only) they might receive 10 eurocents for the inbound call and pay 7 or 8 eurocents for the roaming charge. These providers would not risk losing money on inbound calls because certain users will find and exploit these opportunities (it's known as arbitrage), and that would make the whole business model vulnerable to collapse. CA: SaskTel, Wind postpaid; Rogers, Bell postpaid iPad flex plans; US: T-Mobile postpaid data, prepaid voice; PureTalk (AT&T MVNO) prepaid voice/data; AT&T prepaid iPad plan Hardware: Too much but notably iPhone 5, iPad Mini Retina LTE, Moto G LTE (N.A. version), iPhone 4. All unlocked.

	(#5)
DRNewcomb (Offline) Senior Member Prepaid Professionist Posts: 1,465 Join Date: 27 Feb 2004 Location: Mississippi, USA Country:	14-04-2008, 23:43 They use a GSM function called USSD (Unstructured Supplementary Service Data) which allows the roamer to send a free signal back to his home switch. This signal says to the home switch, "Call me back and connect me to number X" This allows the service provider to control the call costs real-time and not wait for the roamed carrier to send the bill as with regular post-paid roaming. The ideal "free incoming" roaming SIM will be issued from a small country with lax regulation where the carrier can charge a high termination premium. Some of these are special numbers that are charged higher than a normal mobile but many international long-distance carriers just average the costs and charge one rate to all mobile numbers in a given country. The high premium will cover the rate to make an international call to the roamed country plus their wireless surchage, which is all the carriers usually pay. (The normal practice in international roaming is to rip the customer off to the maximum extent possible.)