With any comms app - and there are loads, e.g. Kik, Signal etc. - I think the main thing is to use one that doesn't bypass you knowing the actual phone number of the other person, or else a creepy/dodgy client could just keep signing up with a new user name. Kik is an example of this AFAIK, i.e. you don't need the number, just the user name.
And a general comment: this is an information forum for sex workers, so I don't think it hurts to make respectful, relevant comments, i.e. if a newbie escort was reading this thread, I'd want her to know that IMO it's essential to have a separate work phone. I'd say this is especially the case for younger ones who are addicted to social media and don't realise the outing dangers that they can cause.