Many times once a Sales or Marketing person leaves current job and joins a new company old customers if unaware of change calls you?

would humbly take down the query/lead etc and forward it to his old colleague to follow up from there on..Also would request my replacement to send out mailers to all major clients updating new point of contact

Tell them the new status and you are willing to help them anytime even if they are asking about your x-company products.

I will give him choice first!

If I receive a call from a customer of my old employee and my current is a competitor, I will give the customer the choice!

Before I offer him any of my products/services I will inform him that I have actually left the company and now working with Co. X, if you want I can give you another contact number or I can give you an idea about what my new company offers!

willing or not, a marketing or sales person develops own net of customers, acquaintances, or partners. 

Customers, Partners or Business associates, often develop a friendly based relationship with the Sales or Marketing person on the other side, because of the benefit of the business. Sales or marketing persons develop certain level of confidence between them and customer or business partners. Same like in personal life. So it doesnt  surprise when old customers are calling unaware of the change. Sometimes it can be beneficial to your new position or company.

Lets say that not only the Sale/Marketing person shall be called by the customers of the previous company they have left. Any employee who interfaces with customers when he/she changes the job, its most likely that he/she will still be contacted by the previous customers especially if that person has not given prior notice to the previous customers that he/she moved on. In this case its always prudent that the customers are notified by the Management that such a person has moved on, so that he/she does not deal with the customers of that company in his/her previous capacity.

Agood Sales person who has left the company would take the opportunity to notify the customers who are calling him/her that he/she moved on to another company and advise them to call another number of that previous company to get help.

Explanations of why he/she moved on should not be given.

To be able to avoid this, the management should always notify the customers of the changes that have happened as soon as the employee moves on. After all, no employee can stay with a company for ever. 

A good  sales or marketing person will not make a customer dependent on him alone. A good sales person will have to share the customer with other if not all parties concerned in the delivery of a customer's requirement. 

This is good and beneficial to all parties concerned. The salesman can move on and get more and bigger customers, the customer service and delivery team can learn from the customer and the company as a whole can expect a easier deliverable and most important, the customer will have a feeling of comfort knowing that the team not a singular person is taking care of him.

often yes , because all business built through a power of relations between persons, for these reasons if the sales or marketing person leaves ,most his customers follow him without thinking,maybe someone say the customers need the quality and price before person's , yes but Loyalty may be of persons as a brand

Regards.