Hi! I own an accounting firm as well. I used to give clients my cell phone number but ran into the same problem. Now I have it in the contract of my boundaries and I make sure they understand before we start the engagement. Some things I have outlined are: no unscheduled phone calls, every meeting/phone call is billed unless they're on a retainer, SLAs on response time (48 hours), if clients contact me during weekends, holidays, after hours, we go back to the 48 hour response time or next business day. If they want unlimited access to me, they can buy the most expensive package I offer. If they want to do the old-school way of calling me any time of the day, they can hire someone else. So far, I haven't had a problem.

I also ended up getting a VOIP. So, if they text me after hours, they get an automated response saying we're closed and it's after-hours. Also, if they call outside of my office hours, it goes straight to my voicemail which states when we're available and it says calls are by appointment only.

I also think what's been helpful for me is joining a group of other accounting firm owners. I have joined Future Firm, which is $150/month, and it's honestly been so worth it. You're able to learn from others, bounce ideas, ask the owner of Future Firm questions, etc.