Problems with inbound calls on voip-number

I do not know why, but when I receive an inbound call on my voip-number of voipbuster - on almost 70% of inbound calls - I'm not able to hear the person who's calling me; whilst they are able to hear me.

I do not have this problem on my voip-number of my ISP.

What might be the reason for this problem?

Sounds like a NAT issue. If you provide your configuration here, perhaps readers will be able to spot the culprit(s).

I've a Fritz!Box fon annex A-modem/router.

The details I'd to fill in for my voip were:

user name, password and registrar.

On the router settings I do not see any NAT-settings option.

diquydiq :

I've a Fritz!Box fon annex A-modem/router.

Is this your main router with a public IP address assigned to it? If so, then your SIP configuration doesn't need any NAT and should work without any problem.

Indeed, that's why I do not comprehend the malfunctioning of some inbound calls - people sometimes do have to call manytimes before the connection is well established. Outbound calls are normally processed.

I do believe that the fault might be on the voipbuster server. Unfortunately their customer service isn't that satisfactory.

Like I said before, unless you provide your configurations here, there is nothing readers can help to identify what causes the problem(s) you have encountered with your TA.