Many Factors we need to consider :
1. Costing
2. Maintenance
3. Flexibility ( interms of on demand increasing capacity)
4. Connectivity to Internet (since voice is very critical)
5. Technical support on site.
Other part you need consider advantage with cloud offer, if you have geo location based, that is easy to manage, no on prem device required, due to pandemic, users will be all over so they plug in the device they get Telephone, some cloud provider offer softphone now. that can be any where.
it all depends on use case, but i prefer cloud on this case (if you have large user base - so they can offer both SIP and TDM offers)