Have you tried a return call to a welfare call? You will get a message that is something along the lines of 'You were called by one of our phone card / welfare account holders...' Not all phone networks are PSTN.
When I was in Bastion a few years back our Det had a phone at the back which our wives knew the ext number so phoned the Corsham Operator which is a national rate number and asked to be patched through via the military system.
The only real issue was that when Op Minimize was on it was still able to be used.
As for Opsec, it would be the same case as using the Paradigm phones - don't talk shop