Sending SIP calls via proxies in France reduces latency and improves call quality. An unfortunate trend among call shops is to operate with one node in Frankfurt – but for our users south and west of the German border (and north of the la Manche), this often represents a serious problem in terms of call delivery and voice quality. Your supplier may be "separating media and signalling", as they put it, but that does’nt reduce the amount of hops and network transfer.
France is directly connected to UK networks via a multitude of undersea cable systems,
such as TAT-14, Tamares, CIRCE, FLAG Atlantic, HUGO and more. If you have voice traffic originating in the UK and terminating to areas in the Mediterranean, Middle East and West Africa, route through Paris or Marseilles, not Frankfurt.
The France-IX exchange is directly reachable from both Paris and Marseilles locations, as well as from multiple Interxion, Equinix, Telehouse, Telecity and SFR facilities in France. Interoute, Cogent and France Telecom have datacenters in Strasbourg.