I would respectfully suggest that if you want to hire a Personal Assistant the amount you pay is incidental. Only one of the needs of the your business (answering text messages) are going to be met and it might not even be needed to be met in the first place. If you are "a private" does that mean you are self employed? If so then the you should pay your "assistant" a percentage of the revenue they generate for you and answering phones and picking up coffee or drycleaning and all that stuff is just part of the deal. If they earn you earn.
And you say you don't do social media etc, but then what tools do you use to promote yourself? Whatever they are why not bring this person into contribute and for your own benefit (especially if your paying for it?). As a minimum she must be freeing up your time and not simply be a replacement for what you must currently be managing to do on your own at the moment, but at no direct cost to you? Otherwise why have someone around to help?
If you don't need any of that or if you don't see any earning potential for youself or the person assisting then you dont need an assistant. Spend the money on your business so that the business (you) make money isnt that how it works?
On the issue of phones and text messages consider what might be done differently. Does the situation justify the $25 cost for an employee that doesnt generate any income for you if all they do is answer text messages? Which I would suggest is preaching to the converted: What's the number and how do they know what it is? I would guess it's one you are advertsing somewhere or have it in an ad or is listed on a site or any combinaton of those things. If that is the case then it is one of many numbers similarly listed by your compititon. And anyone texting it are doing so because your telling them thats how they can book your services. Why not just take the booking then and there?
Why couldnt you use something like Office 365 which for around $20 a month is being widely used by individuals, self employed people, professionals and big business to take appointments and book resources and to make appointments and see a persons availabilty and arrange meetings blah blah - all sans phone because those people being booked are busy working! As a customer In your industry I send out more than one message to different ads because I dont know who is avaiable or whatever and I expect that whoever replies to my message for a booking is not the girl I want to make a booking with. Doesnt most people? So whats the point of persisiting with that system? If I could see when your free, make the booking and you would know i am coming over it would save time and be easier and cost us both less! Lol. Anyway sorry that this turned into to a way too long reply and probably hasnt helped answer your question! Regardless good luck and hope everything goes well for you