You got the right idea but doing it by hands is a little anarchic (I have this idea for a while too)

Maybe you should consider those kind of device

https://www.alibaba.com/showroom/multi-sim-card-gsm-modem-8-ports.html

And if you want to push more this idea, with some knowledge of programming (python or php) and some basic of sysadmin linux, it is highly feasible to retrieve the SMS from this kind of gms modem and send them into a server/vps where each customer could retrieve them automatically 24/7, monthly payment to keep up the sims card, if your users decide not to pay you disconnect the sim card... A nice little project, push it more.

One thing, I'm not sure it is legal in the Philippines but if you keep low profile, should be fine