Often caller can hear me but I can't hear them

Maybe 1 call in 10, the caller can hear me but I can’t hear them. If they call right back then sometimes is it ok, sometimes not.

Calls come in from an IAX provider to my Asterisk server then are routed via IAX to IAX soft phones on the local LAN, or to remote Asterisk servers.

The IAX provider offers SIP as an option and I’ve tried that. I’ve also tried SIP soft phones, and even other providers always with the same problem. I’ve also tried many different dialplans but all are pretty simple.

The provider suggested I not use jitterbuffer or trunking, both of which I am not using.

Could the problem be that I have no timer? I have no ZAP card nor do I have ztdummy loaded. Any ideas would be welcomed.

You have answered you own question.
You need a timing source for iax so install ztdummy or a cheap clone.