T38 is your best bet for fax communication through VoIP networks. But, not every VoIP connection has been created equal, nor are all VoIP providers one and the same. Completion rate can be poor, or interoperability wobbly.
You must make careful enquiries about T38 support when shopping for an ITSP or VoIP provider. Ask around, learn which are the proven operators in your region, and do your your research.
It is not unusual for a medium company to use an ITSP specifically for faxes, while using another one for voice traffic. This setup can also double as failover; for example, if one ITSP is down, you will redirect all traffic to the other one.