How can I manage multiple calls when I receive call on my mobile phone & telephone at same time?

You cannot hold if both are important, so its better to answer one properly and if required you can say one will call back and answer the other one,, its not good to keep on hold while answering other call. Choose the better or important and from other one get the number and call back.

1st classify according to periority

receive most important

delegate 

postpone less important

Differentiate with respect to Business priority.

Practically it is not possible to speak on two phones at the same time, so you will have to tell the person on the mobile you will call him back while you fininsh your call with the person on the landline.