Never had to use an excuse as it's always hidden. Being single also means there's no OH snooping around. The only time I gave an excuse was when I was buying the phone, told the shop keeper I was going on holiday and wanted to buy a cheap phone to put a foreign sim in while I was there. The guy was understanding and helpful.
I know it's only the Mrs that we have to be wary of and not MI5 but the guy in the phone shop might remember you if you had a convo with him.
I bought my punting phone in Argos in a different town, went to the self service machine, paid cash collected it when my number was called out. Never registered, always topped up with cash vouchers.
But as Nancy Sinatra never said "and then I go and spoil it all by saying something stupid on UKP!!"