It's a decent idea, and no, it is not illegal.
It could theoretically be done using sim cards - this would not require cloning sims, which is illegal, but rather it would be two separate sims, or extensions if you like, on the same account and with the same mobile number. While not illegal, this would require new industry standards to work properly, which seems unlikely.
However this could be done, and is being done, leveraging voip and wifi calling, with devices being linked under one account and calls made/received on the same number. Giffgaff still doesn't even offer wifi calling, so don't hold your breath about features like this on giffgaff or other MVNOs.
on: 10/01/2019 | 21:13 - edited: 10/01/2019 | 22:05
@tazzy19 @_kara_ correct me if I'm wrong but what's not legal is two sims with the same IMSI number, i.e. cloned sims.
However, two sims with different IMSI numbers that are linked to the same mobile number aren't the same as cloned sims, are they?
Message 17 of 21
I believe that in this case what is happening is that in this case the SIMS technically have different numbers but a network can provide a "Single Number" service where the second SIM can behave as though it is the same number. The second SIM uses "presentation CLI" to spoof outgoing calls and a network-level call-divert to manage incoming calls. I guess this may be possible for someone to set up for themselves? This sort of tech stuff isn't really my area so I can't be certain though.
I guess this may be possible for someone to set up for themselves?
Part 1 -- the ability to ring a second number if the call is not connected to the primary number -- can be satisfied by setting up divert on no reply and divert on not reachable. If both numbers are on giffgaff, then the diversion would be funded by free gg to gg calls where applicable, otherwise it would come out of the member's goodybag. Reasonably practical, reasonably ecconomical and needs no system changes in the short term
Part 2 -- using presentation CLI so that outgoing calls from the auxiliary number show the caller ID of the primary number is not presently possible and would require some development from giffgaff.