I have taken delivery of an additional handset and it did take several attempts - but just press the reset for a second and place the handset on the charging base to initiate the base station search - ok did take about 8 attempts but it did work.
I think pressing the reset just makes the base station 'visible'
Do not hold the reset down too long or you de-register the existing handset too like I did - so I had both handsets to register