Asked to transmit frame type 64,

What does this message in the CLI mean?

WARNING[9641] chan_sip.c: Asked to transmit frame type 64, while native formats is 0x100 (g729)(256) read/write = 0x100 (g729)(256)/0x100 (g729)(256)

What problem are you trying to debug?

The message is a bit strange, because the largest frame type is 20, however it is a warning, so isn’t worth investigating unless there is some real problem to which it may be related.

If you want to know the exact context in which it is used, search the source code for it.