PCM to AMR conversion problem

Hi,
I am working on audio streaming for Nokia 6630 in Symbian 8.0. I am trying to convert the PCM samples to AMR using the following code:

// Uid of PCM16toAMR codec is 0x101FAF68   
CMMFCodec* codec = CMMFCodec::NewL(TUid::Uid(0x101FAF68));
CleanupStack::PushL(codec);
CMMFDescriptorBuffer* srcbuf = CMMFDescriptorBuffer::NewL(320);
CleanupStack::PushL(srcbuf);
CMMFDescriptorBuffer* dstbuf = CMMFDescriptorBuffer::NewL(32);
CleanupStack::PushL(dstbuf);
TCodecProcessResult result;   
srcbuf->Data().Copy(*iStreamBuffer[iStreamIdx]);   //Copy your PCM frame data into srcbuf, for example:
result = codec->ProcessL(*srcbuf, *dstbuf);
(*iEncBuffer[iStreamIdx]).Copy(dstbuf->Data());   //Copy the AMR data into the buffer

CleanupStack::PopAndDestroy(dstbuf);
CleanupStack::PopAndDestroy(srcbuf);
CleanupStack::PopAndDestroy(codec);

Again AMR to PCM conversion i use the same code with UID 0x101FAF67.
I give 320 bytes of PCM sample for conversion but I get only 13 bytes encoded AMR data. Again when I pass this 13 bytes, the AMR to PCM conversion gives 32 bytes which i feel it is wrong. When I play these samples i get a garbage sound. Can anyone tell me where I went wrong? Help needed urgently over this...
I want to know what AMR encoding rate is used.