Who each provider is really for
Servetel
Servetel is generally positioned around business phone systems and call centre capabilities for Indian companies, with an emphasis on getting a professional inbound and outbound setup running without hardware. That shape suits an operations team that wants call flows configured, agents connected and reporting available, without a development project attached.
Exotel
Exotel is usually described as a cloud communication platform with substantial programmable capability alongside its business calling products, and it is widely used by companies that want to build calling into their own applications. That shape suits organisations with engineering capacity, higher volumes, or workflows unusual enough that dashboard configuration alone will not express them.
Confirm current positioning with each provider, since both evolve their offerings.