If you get audio probs and on the CLI you see:
error sending DATA_B3_REQ (error=0x1103, datalen=160) B3in=1
try to increase the "AST_CAPI_MAX_B3_BLOCK_SIZE" from 160 to 320.
To do so edit the file chan_capi_pvt.h inside chan_capi source from:
recompile and reinstall chan_capi, (re)start (*) and test.
Passive cards (aka FRITZ!) use "software buffers" from the main memory of the box to transfer the voice-packets via capi.
Active cards have limited amount of OnBoardMemory in hardware and it could happen that the small voice-packets fill up the buffers inside the memory until a "overflow".
The b1 has only 8 buffers and from AVM I know that they use 256b chunks to transmit via capi.