My Philips Kala cordless phone batteries need replacing. It is fitted
with 2 x AA 1.2v 600 mAh NiCd batteries and the handbook says only AA
600 mAh rechargeable batteries should be used as replacements. I have
some AA 2000mAh Ni-Mh batteries and was wondering if it would be ok to
use them in this phone?

The technical specifications say:
Power Supply
Handset : 2 x AA/LR6 1.2 V 600 mAh NiCd
Base : 8V DC / 250 mA Output 1
6.5V AC/ 95 mA Output 2