I don't chat on the phone at restaurants, but I can address this issue from the customer's POV. Last night, DH and I tried a new restaurant and it had a VERY long menu. The waiter took our drink order and when he brought our drinks, he asked if we were ready to order our meal. We told him we weren't and that we needed a few more minutes. I swear, every two minutes, he popped back over to ask for our order again. I barely had time to find my place on the menu before he'd interrupt us to ask for our order one more time. At some point, I wanted to say, "When you look over and no longer see my face buried in this menu, but instead see it placed on the table with me no longer reading it, THEN I will be ready to order."
I didn't say that. I resisted. But really, if a customer is clearly reading the menu (as is their dining companion) then they are NOT ready to order and rushing them won't help matters. I would think that when a diner puts the menu down, it usually signals their readiness to order.
If they're on the phone, I don't know WHAT that signals, except for them being the person I'd hate to be stuck at a table with.