Let's speculate. It could be an important call and the conscientious rider is wisely walking rather than attempting to talk and ride. Or, it could be a flat with a call for help. Speculating further, it could be a new bike with a pinch flat. Always deflate those tubes, massage the tire, and slowly re-inflate, to let the tube move into the correct position. Why the phone call instead of just making the change? Could be a rear flat, which, for someone unfamiliar with a Sturmey-Archer hub, presents a confusing scenario--how the heck do you get that darn wheel off of the bike? Internally-geared hubs are becoming more popular every day. Here's a link to one video and another video explaining flat repair with a three-speed hub. Watch them, give it a try, and save yourself that embarrassing phone call (not that that's what's happening here). And, do remember to pack a few disposable gloves in your repair kit. You'll be glad you did.