I recently got some C620H handsets to work with a GO box and S850A ... The cheapest way was to buy a triple set with a PSTN base station at just under £75. Singles were £40, twins £70 and the triple £75 although the same supplier now has them at £90. http://www.ligo.co.uk/siemens-gigaset-c620a-trio
They may not be the cheapest handsets but they do feel good and have good overall functionality Ligo also do trio sets of the C430 and C530 which are a little cheaper. As far as I know the very cheap bottom end handsets are not compatible with a GO box..
M H C
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