Spanish phone companies refer to "roaming" as the possibility of using your cell phone outside Spain. If you want to buy/rent a phone in Spain and then use it in other countries you have to ask them (they have to enable the roaming service for you).
If you meant you wanted to travel around Spain with your cell phone, that's ok. You won't have any problems. However, some companies have better coverage in some areas than others.
Currently there are three cell phone companies in Spain:
* MOVISTAR. It usually have the best coverage but their prices are quite expensive. People say it has better coverage in the north of Spain.
* AIRTEL. It has better coverage in the south of Spain but it hasn't in some areas in the north!!.
* AMENA. This is the newest companie. It has the best prices but the worst coverage. However, in some areas it uses the Movistar coverage.
As for calling someone in case of car troubles, if you are in a highway where you have to pay, you need to call the company running (no other companies are allowed to enter the highway).
At any other roads, your car insurance company usually have an assitance service. Of course, you can always call the Civil Guard (which operates in roads).
PS: Visit this official site http://www.dgt.es
(there is an English version).
The best tips from your favourite hostal in Madrid.
Hostal Chelo at http://www.chelo.com