This is something that very much depends on the therapist, and it's something you should talk to your T about. Clarify with her when/how you should reach out to her when you're panicking or need extra help.

My T responds to email and text, but there is often a delay. If I need to speak to her right away or if it's something time sensitive or urgent, we've agreed that I need to call either her office or cell phone. Text messages can fail to send or be overlooked. So, if I were panicking and needed to talk to her, if it's during her normal office hours, I call the office line and leave a message. After hours, I call her cell phone. She might follow up with a text saying she got my message and is free at x time, or she might just call me back. Either way, I know that with a phone call, she's going to get back with me within a couple of hours at the most.