You have to set the BTS attribute
DIRTCHASS
This attribute specifies whether the direct assignment of TCH
channel is enabled or disabled. The meaning of the setting values is
the following:
FALSE: Direct TCH assignment disabled;
SDCCHMS: Direct TCH assignment enabled but
disabled for “answer to paging-any
channel” cause of CHANNEL
REQUIRED message (since
SDCCHonly MS may be supported by
the network);
NOSDCCHMS Direct TCH assignment enabled also
for “answer to paging-any channel”
cause of CHANNEL REQUIRED
message (since SDCCHonly MS is not
supported by the network)