I experienced that on the LG Leon (Android Smartphone) there is a much lower volume when talking over VoIP than when talking over regular GSM. Outside the building with additional noise, a conversation can't be performed properly.
This happens not only with Zoiper but also with Skype or Viber, so I believe it is a general VoIP issue.
On other smartphones like iPhone or HTC ( same PBX environment ), the volume is much higher.
Unfortunately there is no setting or app which could help to increase the volume and the LG support does not even try to help.
Maybe someone here might have an idea what to do?