One of the reasons the associations agreed to FW moving to the en route sectors was that services would not be diminished.
There was also a safety value in cancelling on the frequency you were using en route as you did not lose SA for what was happening by being off freq, and you were on the right freq for a call if something went amiss.
Certainly we should use phone if available as best to NOT rely on third-party handling but as pointed out at the start - phone may NOT be available/reliable therefore it may be better to inconvenience one ATC for 30 seconds than the entire system going into uncertainty phase and a lot more effort.
We all accept that the current (mis) management of ATC staffing has created problems - that is an Airservices issue and should not be allowed to remove the services they are meant to provide.
My suggestion - advise the ATC "facility not available, kindly accept the cancellation".