Independent Network With Roaming to Standard GSM

Suppose you have an isolated area with no cellular coverage. You want to cover it with OpenBTS. Not far away there is a city covered by a standard GSM carrier. You want a phone that will work in both places with a single number. You do not expect to negotiate a settlement agreement or direct network integration with the incumbent GSM carrier.

1. Get a GSM account and SIM from carrier in the city.  This account will include an E.164 DID (a "telephone number").

1. Get an account with a VoIP carrier, including another E.164 DID.

1. Set the "Call Forwarding on Mobile Subscriber Not Reachable" on GSM account (from step 1) to point to your VoIP E.164 DID (from step 2).

1. Program the SIM's IMSI into your OpenBTS Asterisk server.

1. Set the network preferences on the GSM phone to favor your OpenBTS system.


  • When you take the phone to the city you will get service from the standard GSM carrier.
  • When you are around your OpenBTS system, you will get service from that and calls to your "city" number will get forwarded to your VoIP account and from there to your OpenBTS Asterisk server. To the outside world you appear to have a single number, the number from the GSM account assigned in step 1.

Also, if you are really cheap, you can just get a single E.164 DID in step 2 to be shared by all subscribers through a set of extensions in you OpenBTS Asterisk server, but this will make dialing non-standard for inbound callers.

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