To be fair, there is a portion of Spanish / French airspace in that area which is very bad for VHF comms, i.e. it shouldn't be, but it is ?!
Accordingly, it is perfectly feasible that an airliner might suffer a temporary loss of 5-by-5 comms with the previous centre, and thus find themselves somewhat surprised to not be in comms with the next FIR, and thus with a fighter escort in tow.
That said, it would probably be fair to say that (unfortunately) many pilots no longer routinely monitor 121.5 on box two and / or indeed have the relevant Jepp en-route chart to hand (upon which 'en-route' frequencies are readily accessible)
Ps. Hindsight, as they say, is a wonderful thing !