Pretty vague question. Google the phone number and see what it returns. Maybe it is your internet provider, maybe it is your electronic payment provider(paypal or the like), maybe it is the owner of the guest network, maybe it is a scam. If it doesn't provide you back with any recognizeable feedback, I certainly wouldn't just pay the fees.
No update from user yet. Wasn't sure if the message said that the guest network was locked, or if the actual guest network connection was indeed locked/denied access and someone asking for payment to unlock it.
Has all the indications of a scam. I certainly wouldn't send them any money, and in fact would not even call the phone number. If the Roku can't be taken out of guest mode, someone with access to the Roku user account has likely changed the password. I would alert @RokuDanny-R and create a new user account and reset the Roku to the new account.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
I would alert @RokuDanny-R and create a new user account and reset the Roku to the new account.
That's FACTORY RESET, accessed via Settings > System > Advanced system settings > Factory reset. The unit will restart and have to be set up from scratch, as though it were a new unit just taken out of the box.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T