Hi!
Where are you from and what is your carrier?
I am from Croatia, I am on T-Mobile.
I am glad that you replied.
This is frustrating and big problem for me. One thing I can tell you - It is not hardware problem!
Of course I contacted HTC, send over 15 emails, they suggested to take phone to the service to HTC

. When ever is some problem with the phone they always say take the phone to the service
I was insane and I have been testing my Sensation XE for about a week and I came to the following result:
This "mute on answer" was happening to me only from certain numbers:
For example:
1. My wife calls me and every second call I am unable to hear her for 5 seconds!
2. My friend calls me and everything is working normal - I can hear him every single time.
So there is a lot of numbers which I hear every single time right away and there is also a lot of numbers which I hear every second time.
I tried 4 different SIM cards from T-Mobile and the problem was still there with every SIM.
Nex thing I did:
I bought a pre paid SIM from another carrier (VIP) and there was no problem at all. Everything is working norrmal.
So I called my Carrier service department, spent about 3 hours over the phone that day, they called me day after - spent another 2 hours over the phone, next day another 2 hours and they managed to solve the problem partially! The "call delay" is now about 25% of all my calls.
T-Mobile service said that they will resolve my problem (I hope) but they have to do some more things (I don't know what). So now I am still waiting for them to call me and to say that they have fixed the problem.
As a conclusion: The problem is somewhere in the phone Radio ROM, Sense UI and kernel when registering on some networks it causes that problem.
How about you? Have you called you Carrier service or HTC?