The problem you are describing could be caused by the adapter or the OPIS handset being defective. Hopefully the following questions / suggestions will help to zoom in on the cause:
1) In order to make sure the OPIS handset is working correctly please test it with another mobile phone providing a standard 3.5mm audio jack.
2) Possibly you are not using the original apple lightning adapter. Please note that some second source adapters do not detect the attached device correctly or may not even be detected by the iPhone.
3) Please note some (second source) adapter support only speaker but not the microphone. They are intended for listening to music only. Please check if another standard headset works as expected with your adapter.
Please note that the adapter provided with the OPIS handset (3.5mm female to 3.5mm male) is only needed for a few phones manufactured before 2011 (e.g. Nokia) following the OMTP standard. If used with current phone the adapter will cause audio disturbance. So, do not use it with your iPhone!