no it is not necessary but I recommend it;
CCNA Voice is nice intro into Voip world; you will have easy time understand some topics in CCNP VoIP , which is 5 exam long;
You can test for any of the CCNP Voice exams without having a valid CCNA Voice certification (pre-requisite) since Associate and Professional level exams do not have to be taken/passed in any particular order.
However, please note that you will not be certified in CCNP Voice until you have fulfilled all requirements- valid CCNA Voice + passed required CCNP Voice exams.
As Martin stated, you will have 3 years to complete all of the exam requirements for CCNP Voice. The three years starts as soon as a candidate passes the first exam required for that particular certification. Again, you must also have a valid CCNA Voice certification to fulfill the pre-requisite requirement (or any valid CCIE certification can act as a pre-requisite) to become certified in CCNP Voice.
The CCNP Voice certification requirements can be viewed here.
Cisco Learning Network Moderator