Iphone 3gs signal problem

hi did a search on this and could not find a fix so apologise if somebody has already had a thread on this.

brought a 3gs off ebay as faulty..

now people can ring the phone and its fine for incoming calls but the discription states that there is a signal problem when they try and phone someone.

is this a possible software issue or does it sound like something possibly worse

..

btw thanks in advance for any advice

 

thanks you. i will do that as soon as i get it. thanks for your input