Audio Encryption in Voip SIP
You won't be able to use a stream cipher with VOIP, because packet loss will prevent you from properly decrypting data that follows the missing chunks.

It may be possible to work around this issue, if you can figure out the size of the missing data, and assuming your chosen stream cipher does not have feedback, fast-forward to cipher to the start of the next block, all at the cost of wasted computation.

The correct approach that I am aware of, is to use a block cipher in CTR mode of operation, where the counter is based on the sequence numbers stamped in the packet headers. Essentially, you need to use a cipher that supports seeking within the data, without needing to decrypt all blocks prior to the block of interest.