As a part-time lecturer, this is the first time that I have been assigned to drive a bachelor's thesis. I try to convey to them my interest in their project. And, I also recommend them to be in contact with me. A student asked me for my personal phone number, and I gave it back. We have exchanged a few WhatsApp messages, always, fully work-related. Nevertheless, I have serious doubts about if it is professional enough to use this means of communication. And if I should avoid this in the future.
Giving your personal phone number to a Bachelor's Thesis student will allow the student to contact you at inconvenient hours or during your free time. This is the main reason why people do not give their private phone numbers at work (neither to students nor to their boss).
If you are fine with this, there is probably not going to be any other major consequence. The student is not going to think less of you (on the contrary, they will appreciate it), and others (colleagues, boss...) will probably be indifferent. In the end, it is your choice how you want to communicate with your students.
I would recommend that you avoid this in the future. At my institution it's officially highly frowned upon -- in fact, the school strongly leans on us to only have contact through the official school email accounts, so that if disputes arise (harassment claims?) they can track down and look at all the contacts.
To me, the main thing is that it potentially impinges on your personal and free time which must be defended at all costs to avoid burnout. Make sure you're tracking your actual time spent on tasks carefully so that doesn't spiral out of control. I recommend that you give clear contact expectations to any students you have contact with (e.g., "I usually respond to email within one business day.").