neither party could connect when invited to speak

hi all,

xp pro sp2 using window messenger 4.7 (not msn), T1, behind routers,

although i am able to use window messenger to 'chat' (i.e. using the
keyboards) with the other parties, neither of us could establish an audio
connection irrespective of who issued the invitation (we can see each other
invitation and acceptance). additionally, we are also not able to receive
file send by the other (although we saw the transfer notice), regardless how
small it is (e.g. even a simple notepad.txt file, 1 kb). i don't know whether
the two are related but thought i mention them anyway in case it may be
helpful to help me.

thank you all for your help.

jes