I don't think this is a problem with Axon, it's probably an issue with the phones.
Some IP phones have a gain setting for volume in and out. What might be happening is the volume out gain is low so when calls are made to customers they can't hear you. For internal calls it could be a case of the call comes in soft and the phone automatically boosts the volume with gain-in control, so you don't notice.
It could also be a problem on the VoIP provider end of things. This would also explain why internal calls seem unaffected, but calls to external parties have the problem.
To confirm this, try calling out from one of your phones to your business number and then answer with another one of your phones (thus making sure the call is sent out externally and then back in), see if the volume is soft. If so, it is definitely a problem with the VoIP provider, if not I think it's the phones.
