GSM or CDMA cellphone for telemarketing (ers)

Today, I've received 4 phone calls from AirAsia - HSBC credit card salesperson.
They were using these numbers +628138370xxxx

Few weeks ago, I also noticed that salesperson and marketing person of Indovision also using Telkomsel cellular phone number.

Asking here and there and googling, perhaps these companies are using some GSM/ FWT gateway.
GSM: Global System for Mobile Communications
FWT: Fixed Wireless Terminal

Basically, these devices allow you to redirect your call through the numbers of the same carriers (operators) so you can save some fee because normally, calling a number from the same carriers should cost you less.

From Amazon, this is how they look like:

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