When receiving a call on my gigaset al18h I can hear the person talking but they cannot hear me
The problem can be in two vertients:
1.- You have the phone in "mute" position, then nobody can hear you, this is easy to identify looking at the lcd scree on the phone for an icon MUTE or the symbol of a microphone with a transverse line on it.
2.- The microphone piece inside the handset is broken or one of its 2 wires is desconnected or desolded, you can try to open the handset but if you do not have any idea what are you doing, don't try. You can cause a worse damage in the process.
I think the second choice is more likely your issue, since the MUTE function resets itself when you hang up a call.