Not sure if Chat is any better but I am running an AI bot over SMS currently and it is wildly more expensive to SMS than it is to actually process the conversations.
I spend about $0.30 - $0.50 per day per user (8 - 10 messages). Twilio is around $0.08 - $0.10 per message to send because the fee isn't per message but per segment. MMS without doubt will explode that cost.
Not to mention if you go on SMS, you have to pay $10+ plus a monthly fee to "validate your identity" plus the ~$1 per month per number.
This is entirely before you discuss platform risk especially with Twilio wanting to go mainstream and investor-friendly. All it takes is one news story and Twilio starts kicking people off platform for "adult content".
If I was designing this kind of system, I would probably just use like websockets or some other out of the box chat functionality. You'd basically be in the same privacy situation as you are right now (creator can see what you send and receive).
If you want to pursue this as a full-on business, you should consider upselling into 'private instances' or other privacy-increasing measures. I'd imagine you'd find people willing to buy the "v1" for hundreds of dollars + hosting costs if it was good enough.
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Oh, and just wanted to encourage you to ignore the "why Telegram".
It's fine. Your audience isn't really HN. We are just here for the tech and startup discussions. Your audience is the 99.999% of people who will go "Oh sick, an AI girlfriend right where I already am!"
I've tried to build for HN-like audiences and it sucks because 1) most of them will just build it themselves or want to run it themselves (ie: not friendly to closed source SaaS) 2) they will not give up their data at all costs (for good and valid reason).
Just write HN and all tech people out of your ICP unless you are selling B2B or prosumer and even then you'll probably not want to sell directly to them but to their bosses.