Is it legal for an employer to get text message transcripts from an employee's personal cell phone?

As a general matter, no--an employee may not retrieve messages (or emails) from an employee's personal (not work provided) cell phone. (If it was a work phone, they could do this, just as an employer may see messages, emails, web-browing, etc. done from a work computer.)

There could be an exception if the messages are somehow going over the employer's office network or server--the employer could then retrieve them from its own system. But if there is no involvement of the employer's systems, equipment, etc., there would seem to be no basis for this.