My 950XL has just started having a similar problem where I put it on speaker and I can hear the caller, but they can't hear me.  Then I turn off speaker and they can hear me again.  If I try speaker (on same call) then it sometimes works where we can hear each other.  When I turn speaker off again, then they can't hear me and I have to put speaker back on.  If I hang up and dial again, then all is good until I try to use speaker again.  Not sure if that made sense, but I've just started having problems with speaker phone function as well.