Hey OP,
If you have real SIM Cards please let us know which country it is from , based on the country verification service can be anywhere between 80 cents to $2 for some big social media sites and for gmail and other it will be as low as 20 cents or less depends on your vendor who you trying to sell and demand
Upto 5-8 Gmail accounts can be verified on one number.
you can redo the verifications on the same numbers after few months for almost all the websites ( FB, Gmail etc )
There are few people who need this service regularly try to search the forums here and you will find your contact
and for people who says it's useless just ignore it ( I used to make 1000$ a week on this kind of services previously )
PS : Since your just a newbie here try to build credit to your profile by associating with established members and offer them few verifications to prove your authenticity .