There is no need for a maulvi to conduct the nikah.
Here's the situation I have in mind: The bride and groom are present at the same place, and so is one witness. The other witness is on the phone; he can hear everything, and all the three present can hear him. Furthermore, the witness is not an unknown person. The people present know him well by his voice.
Okay, I agree with brother jawaid that the ideal situation would be that both witnesses are present where Nikah is taking place, but what if acouple has already done a nikah in the above mentioned-situation and have lived together as husband and wife?