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Is it possible to make ur own gsm antenna that can work just like a regular Mobile antenna? Answered

So that you could just switch network and call for free via that antenna?


No. The antenna is not the issue. The frequency band used by the phone device itself, the protocol it uses to communicate through that frequency band, and the data generated by the phone and transmitted using the protocol are all part of "GSM" vs. "CDMA" vs. "TDMA" etc.

You need a GSM compatible device, which has been programmed with the necessary identification and authentication data from the network provider, in order to use it.

Creating such a device yourself, without participation from the network provider, constitutes fraud.

The closest you could come, I think, is to get a smartphone that will also do VOIP across an available WiFi connection. Of course you then wind up paying for the VOIP service.

The different networks know whether or not your phone is connected to them.
The network hardware is only a part of the service.