Sudden price spike on Spanish number?!

This is not something I'm trying to debug or get support on, I'm really trying to wrap my head around what could have happen. Also, their bot is saying "we can change prices at any point" I'm quite confused.

I have been paying 2.25 every month for the number I have in my account, and suddenly 3 days ago I get a charge of 6.75 for that same phone number.

I haven't had any usage, and I don't see anything weird (no calls nothing) in the summary... Is this something that just happen to me, or is there some "phone is idle fee" or something I'm missing?

It's a +34 91 XXXXX "SPAIN Local Phone Number" with Voice and Fax.