1st call goes straight to voicemail, second one connects? New iphone?

My girlfriend just bought a secondhand iphone 4s which is almost brand new. But ever since she got it, i have to call her twice to actually be able to talk to her. On first attempt, it goes straight to voicemail, and on second attempt, i can hear the dialtone.
its costing double and also an inconvenience.
is it a problem with setting, or anything else. She had a vodafone sim on that phone for a few weeks, same problem, now with TPG. Still same problem.
can anyone please help?

If her 4S has the newest update, it has a feature called "do not disturb" that hold calls on the first call, but will push them through (if you want it to) if you call another time within three minutes. Here are the steps to turn it off: slide to unlock> settings> go down 5 rows, and you should see a moon symbol. Next to the symbol, you will see "Do Not Disturb" if the oval next to these words say "on" and is blue, tap the blue oval to turn it off. Keep in mind this only works if she has the most recent update! Hope this helped!

Been having this same problem for 2 months with my wife's iphone, this just sorted it in 2 seconds, thanks for posting!

Thank you that worked!

Also sorted my prob thanks

Unreal… I've been suffering with this prob for weeks, ever since i downloaded latest software update. Smh.

Thanks, solved my problem too

Embarrassing but I think I must have turned it on my self and was blaming the carrier.

Thank you, Your a Rock Star!

Thanks so much. You really helped.

Thanks that works great

Brilliant, been really annoying me.

Brilliant, been really annoying me.

Have followed these steps. Calls still going into voicemail.

OMG Thank you! IT HAS BEEN DRIVING ME NUTS EVERY TIME I CALL MY WIFE. I WILL TRY THE FIX

This worked so Thank you very much!

Thanks so much. I almost called my service provider