ServiceM8 Phone uses mobile data (i.e. mobile internet) to make & receive calls, so when you're working in an area with low network coverage, the call quality can be affected.
For those times you're in a low-coverage area, you can try managing the call experience with clients by switching ServiceM8 Phone over to the standard phone network.
To do so, in the ServiceM8 Phone app tap More > Call Mode > Standard Phone Network:
TIP: remember to switch the Call Mode back to VoIP with Mobile Data when you're back in a high-coverage area.