Telegram. E-mail. Discord. TeamSpeak (Only know one who uses this). Wickr. I use all but WhatsApp and TeamSpeak, mostly because they're more difficult to handle, there's not as much customization as I'd like.
My first ever thread was at <<snippet>>.org, took about 4 months for the first ever deal to go through, it was 2875 USD, this was in 2013. Then once that went through news sort of spread and within the next 2 months I made 2 large deals, one on the 400k and the other at 700k and about 70something much smaller ones. Was different back then, lots of people couldn't get crypto because of where they lived etc etc.
Depends on what you offer, most brokers ask for ID verification and that doesn't attract a good chunk of potential clients. I don't ask for ID verification so that has helped a lot, you can charge those clients more.
Lots of OTC desks have bought at lower prices and sell at higher ones, that's how they make money. You need to be able to execute good trades, so for example if someone hands you 500 million you can't just log into your Binance account and market buy 500 million, the slippage in this case would be crazy, at 1% that's a 5 million loss.
Just an OTC desk won't be profitable enough to bother with it, you should run a fund too. Anyways, to end this here as I don't really have time and it would take hours to write all that, Bitfinex the exchange would be your best friend. Read this:
Bitfinex | Corporate & Professional, start from that page.