Avoid the 'drop-in' call
As a salesperson, you have the right to be persistent and to be respected, but not to infringe on customers' time.
Source: Marketing executive Ken Thoreson
Dropping in on a client unannounced because you were "in the neighborhood" sends the wrong message.
Instead, schedule your calls, have a stated objective for each meeting, and be sure the time spent results in value for the client.