Hi, I have a set of Phillips Dect phones and the manual says 600MaH. I have just put 1000MaH in them and they are fine. As Chuck says they do take a little longer to charge but then they last longer between charges too.
They are on charge all the while whilst not in use so the longer charge time shouldn't be an issue. Presumably there will be a longer talk time as well?
In the instruction manuals for the last couple of sets of these phones I have had it has said not to have them on charge the whole time as it can reduce battery life. Not sure if this is true. On mine it says to leave go flat occasionally to improve battery life. And in answer to the other part of your question, yes it should make talk time longer.