Will Yoigo SIM work with a GSM phone?

[inquisitor]

Generally UMTS SIM cards are fully downward compatible to GSM, so a Yoigo SIM card should work in your GSM phone. But the question is, whether Yoigo SIMs are admitted everywhere to the movistar network. E.g. In Germany, where O2 has a unilateral national roaming agreement with T-Mobile, O2 customers can only register on T-Mobile in rural areas, where O2 has very little coverage. Maybe Yoigo has something like this in Spain, too.
Petkow will surely know and answer soon.