Message the customer from the job card
Every job card has Message under the main action. Open it and tap a quick reply — On my way, I'm here — where's the vehicle?, Running about 10 minutes late — or type. The customer gets it as a text and on their tracking page; dispatch sees it too, so you don't have to tell them separately.
When the customer replies, the card shows a count and your phone buzzes (if notifications are on).
Keep it to the job
Where the vehicle is, gate codes, ETA, who's meeting you. Prices, extras and cancellations go through the buttons on the card, not messages — the customer has to approve an extra on their phone before you do the work, and that only works through Propose an extra. See Working a job.
No signal
Messages need a connection. If you're in a dead zone, call the customer from the card instead; the number is right there. Your status taps and photos still queue and send later — see No signal.
It's on the record
Messages are part of the job's paper trail. That protects you: "I told the customer at 2:14 that the car needed dollies" is right there if there's ever a dispute.