Hi Itifoski,
Please clarify exactly what you mean here. I am going to assume you are talking about a GSM gateway that integrates in with 3CX. If that is the case you want to look at a beroNet GSM Gateway. This provides upto 4 sim card slots (if you buy 2 x modules for the unit):
https://www.beronet.com/products/voip-gateways/gsm-gateway/ and is fully supported for integration with 3CX Phone System.
There are other GSM gateway options available out there but none that are "supported" with 3CX.
On that note there are also other SMS type online services and products out there available that will send bulk SMS messages for you however I have no experience of these with 3CX.
What type of GSM gateway do you have ? If it is not a beroNet then you will be stuck on 3CX Support they are the gateway of choice:
https://www.3cx.com/community/threads/list-of-gsm-voip-gateway.30049/
There is no other module/gateway or anything for GSM. That being said it does not mean you wont be able to integrate a 3rd party device however it would be a manual configuration without any guides or assistance.
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