Ask HN: App to leverage for multiple conversations?

Not even sure the best way to ask this.

Goal: I have phone numbers in the USA that people have to reach them when travelling abroad.

I want to offer receiving SMS on that number too. I can then SMS it to them, but besides for that costing money, it may confuse them: hitting "reply" will go via the international carrier at large fees.

I can send via email, but people want push notifications...

Is there a way to leverage, e.g. a telegram bot or something similar? The main issue is that I need the user to be able to respond, and that I know which person they are responding to.

Do I just need to make a whole app for this? Some sort of chat/message clone might be a good start... I've only made web sites before, no apps. I don't want to waste too much time if something is already "good enough".

Thanks!!!