How competitive is your market, and how many other companies are running the $350 large adds?

In general, if you are looking in the phone book for a service, you pretty much know exactly what you are looking for. So as an advertiser you don't necessarily have to 'stand out'. A big eye catching add will get a prospects attention but its likely that the prospect is looking for specific information(ie location) so they are likely to look at numerous adds, big and small, before they pick up the phone.

Of course, if there are 40 other businesses you're going to have to distinguish yourself from the crowd.

Another thing to consider is how much of your business is dependant on phone calls.

Hope that helps a bit.