mixed calls ?

Hi everyone,
supervisor 1 calls client A and transfers it to external number 1122334455.
supervisor 2 calls client B and tries to transfer him to external number 1122334455.
upon hitting transfer button (call with customer button) supervisor 2 hears supervisor 1 talking to called person at number 1122334455.
what could be the problem ?
$version = '2.0.4-152';
$build = '81012-0857';
asterisk-1.2.30.2
zaptel-1.2.27
libpri-1.2.7
supervisor 1 calls client A and transfers it to external number 1122334455.
supervisor 2 calls client B and tries to transfer him to external number 1122334455.
upon hitting transfer button (call with customer button) supervisor 2 hears supervisor 1 talking to called person at number 1122334455.
what could be the problem ?
$version = '2.0.4-152';
$build = '81012-0857';
asterisk-1.2.30.2
zaptel-1.2.27
libpri-1.2.7