Hi,
You can its called Call Waiting (CAW), also on digitals the FCN key ODN2 allows the same function.
You need to ensure the Common CAT is set to allow reception of Call Waiting Tone. Look at SERV Digits 3&4 and Digits 7&8...
e.g. CAT=1,SERV=xx02xx07xx,
This allows reception of CAW and initiation... When you call a busy extension press CAW digit in UK this is 5...
Also look ar SERV for the Generic extension to test with DECT.
Regards
Isinor